{'fraction_non_alphanumeric': 0.029576152771308803, 'fraction_numerical': 0.02072659524918491, 'mean_word_length': 5.153295128939828, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Origin of the isthmus? A comparison of the brains of lancelets and vertebrates

Despite recent claims to the contrary (Holland, 1996; Holland and Holland, 1999), there has been a general consensus for over 100 years that lancelets do possess a brain, as constituted by the rostral neural tube, encompassing at least the first six pairs of nerves (Huxley, 1874; Rohde, 1888; von Kupffer, 1906; Bone, 1960; Guthrie, 1975; Jefferies, 1986). Lancelets, also known as amphioxus, are small marine invertebrates, some 1–8 cm in length, that occur in the sandy floors of bays and lagoons. One might ask what is significant about their brain and why its organization is of interest. The answer: because lancelets are believed to be the closest living relatives of vertebrates, and their brain organization, therefore, can reveal insights into the early evolution of vertebrate brains. There is a proviso attached, however: not all lancelet brain features can be considered primitive, just as not all vertebrate brain features can be considered derived. Lancelets and vertebrates have not shared a common ancestor in over 500 million years, and the brains of both groups have evolved in that time. The brains of living lancelets, as well as those of living vertebrates, are almost certainly a mixture of primitive and derived features, whose identity can only be determined in the context of an out-group analysis involving other closely related taxa (Wiley, 1981; Northcutt, 1984). There has also been agreement that the brain in lancelets is divided into a rostral division, usually termed the cerebral vesicle, and a caudal division, but information on the organization of these divisions has been slow to accumulate, and until recently, most studies have concentrated on the cerebral vesicle. By the mid 1970s, researchers had recognized a rostral pigment spot, representing a putative visual organ, and a so-called infundibular organ (Fig. 1A) within the cerebral peduncle of lancelets (Barrington and Jefferies, 1975). The infundibular organ had already been suggested to be homologous to the flexural organ of vertebrates (Olsson, 1956, 1993). The flexural organ is responsible for the production of Reissner’s fiber in embryonic fishes before the subsequent secretion of this fiber by the subcommissural organ in adult fishes and other vertebrates (Fig. 1B). Definitive characterization of the rostral pigment spot as a visual organ did not occur until 1994, however, with the elegant ultrastructural studies of larval lancelets by Lacalli and colleagues (Lacalli et al., 1994; Lacalli, 1996). In these studies, the rostral pigment spot, now termed the frontal eye, was shown to consist of pigment cells, receptor cells, and neurons (Fig. 1A), whose organization is consistent with the interpretation that the frontal eye is homologous to the retina of vertebrates. These studies also corroborated Olsson’s interpretation of the infundibular organ. In addition, Lacalli and colleagues detected a new set of mechanoreceptors, termed ciliary bulb cells, which may function as an organ of balance. Presumably, the ciliary bulb cells have no homologue among vertebrates. Lacalli et al. (1994) were also able to confirm that the cells of the lamellar body in lancelets were identical to the ciliated photoreceptor cells of the pineal organ of vertebrates and suggested that these structures are homologous. In a subsequent study, Lacalli (1996) demonstrated that a small number of neurons (tectal cells, Fig. 1A) flanking the caudal pole of the lamellar body of larval lancelets come into contact with the axons of the receptor cells of the frontal eye and suggested that their position and connections were consistent with their homology to cells of the optic tectum of vertebrates (Fig. 1B). Therefore, Lacalli concluded that what has been traditionally referred to as the cerebral vesicle in lancelets should include the region of the neural tube that comprises the lamellar body. In this case, the expanded cerebral vesicle could be considered to be homologous to the diencephalon of vertebrates, and it is bordered caudally by a short transitional zone that might be homologous to the midbrain (optic tectum and midbrain tegmentum) of vertebrates. This interpretation gained support when Williams