448 THE CH.ETOGNATHA. 



their origin and fate has not been definitely determined. According to Selys 

 Longchamp, they have a distinct longitudinal, usually spiral striation, and ap- 

 parently a small eccentric nucleus. When alive they are flexible, but become 

 hard and brittle when fixed. Their color and appearance is suggestive of mus- 

 cular substance. Similar cells occur in Lingula (Yatsu) and are said to occur also 

 in the annelids (Nereis). Their significance in the annelids is not apparent. In 

 the arthropods they are associated with special forms of muscle building and with 

 muscle degeneration. They are significant in the acraniates (phoronida and 

 polyzoa), for they are indicative of the occurrence of extensive muscular degenera- 

 tion, and of the secondary character of the anatomical structure of these animals. 

 Selys Longchamp figures a deep infolding that is formed in the middle of 

 the haemal surface at the close of metamorphosis. It is not described in detail, 

 but it appears to be the seat of extensive degeneration, similar to that in the 

 cephalic navel or "dorsal organ" of arthropods. (Fig. 305, G, c.nv.) 



IX. THE CH.ETOGNATHA. 



The chaetognatha are clearly to be regarded as the modified descendants of 

 primitive arthropods, for they retain, even in the adult stages, some of the more 

 important characters of that group. The head (Fig. 306, A.B.), with its rudimen- 

 tary appendages, mantle (prepuce), and organs of special sense, represents the 

 modified remnants of the nauplius; while the trunk, which is a voluminous but 

 very simple caudal outgrowth from it, represents the imperfectly developed tho- 

 racic and abdominal tagmata, /, Brc., and ab. They show no recognizable divis- 

 ion into metameres, and are used mainly for locomotion and for the retention of 

 sexual cells. There is no distinct larval stage, no period of fixation, and no 

 striking metamorphosis; the adult is to be regarded merely as a sexually mature 

 naupula adapted for, and leading throughout its whole life, a pelagic existence. 



Development. -The eggs are relatively large (2 mm. in Sagitta), transparent, 

 and contain a considerable amount of yolk. They are discharged in the early 

 morning, and develop very rapidly, the young escaping from the egg membranes 

 between 6 and 8 o'clock on the evening of the same day. Cleavage is total and 

 nearly equal, resulting in the formation of a spherical blastula with a small cleav- 

 age cavity. The so-called " gastrula " is in reality a typical teloccele, and is formed 

 at the caudal end by the infolding of the teloblasts and primitive germ cells. Two 

 primitive sexual cells may be recognized at an extremely early stage on the posterior 

 neural surface of the blastula, D, g, in a position that corresponds with their early 

 location in many different arthropod embryos. (Lernaea, hymenoptera, coleop- 

 tera, etc.) 



The apical infolding carries with it the mesentoblasts and germ cells, the latter 

 finally lying at the anterior extremity of the teloccele, E. With the closure of the 

 telopore, which appears to take place on the posterior, neural surface of the embryo, 

 the mesoderm, endoderm, and germ cells begin to separate from one another. 

 The endoderm forms the walls of the primitive gut, F,en, and the mesoderm forms 



