CHAETOGNATHA. 675 



band of ciliated epithelium behind the eyes which. is disposed in a ring 

 varying in shape in different species, and perhaps olfactory in function ;. 

 a variable number of tactile organs scattered over the body, tail, and fins, 

 formed of a central row of cells bearing sense-hairs, surrounded by sup- 

 porting cells. 



The alimentary canal consists of a stomodaeum, or pharynx, lined by 

 a single layer of epithelium, and provided with a thin layer of striated 

 muscle fibres running dorso-ventrally, followed by an intestine (archenteron) 

 lined by a single layer of ciliated epithelial cells, some of which appear to be 

 glandular. The anus is ventral, and placed where the body and tail join. 

 The intestine is suspended by dorsal and ventral mesenteric bands which 

 are fenestrated, and divide the coelome of the body into a right and left 

 half. The coelome is roomy, and lined by a layer of epithelium not 

 visible in all places. The tail also contains a coelome, similarly divided 

 by a vertical partition, which appears to originate in a backward extension 

 of the archenteron at the period when its original cavity is obliterated, and 

 before it has acquired its permanent cavity (Hertwig). 



The Chaetognatha are hermaphrodite. There are two ovaries lying 

 one in each half of the coelome of the body. Each ovary has a duct in 

 which sperm is often found. It opens laterally on the dorsal side of the 

 lateral fin, where the body and tail join. Two testes, solid cellular ridges, 

 lie, one in each half of the coelome in the tail on its outer wall. Masses 

 of cells are set free into the coelome, and there differentiate into spermatozoa. 

 The cell masses are in constant motion owing to the presence of cilia on 

 the coelomic epithelium. The spermatozoa are filamentous, transversely 

 striated in Spadella cephaloptera. They escape on each side by a narrow 

 ciliated canal, which has an internal ciliated funnel, and ends in a dilated 

 vesicula seminalis. The latter has an external aperture in the posterior 

 third of the tail. 



The ova are enclosed in a membrane, are transparent, and float on the 

 surface of the water, except in the case of Sp. cephaloptera, which attaches 

 them by a gelatinous peduncle to algae. Segmentation is total. There is 

 an invaginate gastrula, and the coelome is formed as a right and left 

 enterocoele. The permanent mouth is formed at the opposite pole to the 

 gastropore, which is closed. There is a stomodaeum ; the mode of formation 

 of the anus is unknown. The sexual glands are derived from two cells of 

 the archenteron, which differentiate early, and divide each into two, an 

 ovarian and testicular cell or rudiment. 



Spadella Marioni, Gourret, An. Mus. Nat. Hist Marseille, ii. 1884-5. 



Die Chaetognathen, O. Hertwig, J. Z. xiv. 1880; cf. Die Wurmfauna von 

 Madeira, Langerhans, Z. W. Z. xxxiv. 1880. / Chaetognathi, Grassi, Monograph v, 

 Fauna und Flora des Golfes von Neapel, 1883. 



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