INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 705 



of the Cliastoguatha, on which Dr. Hertwig's general conclusions are 

 based. 



The more or less cylindrically shaped body of the Chfetognatha 

 is distinguished from that of all other Vermes by the possession of 

 lateral fin-like appendages, and by the special armature of its head. 

 The lateral fins vary in number and character in diiferent species, but 

 the unpaired caudal fin is always constant ; this latter has a general 

 resemblance to that of fishes, but diifers from it in being horizontal 

 and not vertical. The special armature which gives the name to the 

 group may be regarded as consisting of (1) spines or (2) prehensile 

 hooks ; the former are small, straight, and conical, and are arranged 

 in 2-4 rows around the mouth. The latter, of which there are only 

 from 8-10, are very much longer, sickle-shaped, and with a sharp 

 point. Connected with these organs is the special and characteristic 

 apparatus to which Krohn gave the name of cephalic cap. This con- 

 sists of two thin folds of integument, which arise on. the dorsal side of 

 the head, and thence pass on to the ventral surface. The ca3lom 

 varies in capacity according to the size of the species, but is always 

 divided by two thin transverse folds into three parts or segments, 

 which may be respectively denominated the head, trunk, and tail 

 segments. 



The enteric canal takes a straight course through the body ; the 

 mouth is a longitudinal cleft about half as long as the head ; in the tail 

 segment the enteron is represented by a caudal septum which divides 

 the cavity into a right and left portion. There are no kidneys, heart, 

 or blood-vessels ; each individual is provided with two ovaries and 

 two testes ; the former occupy nearly the whole of the trunk-segment, 

 while the latter are developed in the walls of the cavity of the caudal 

 region ; one segment is, therefore, male, and another female. 



Of the external structures we can only notice the fins and the 

 glandular cells. The fins are reported to be made up of a gelatinous 

 supporting substance, of homogeneous filaments, and of an epidermal 

 investment. The first of these is completely structureless and devoid 

 of cells ; its flat surface is covered by the homogeneous filaments, 

 which are closely applied to one another and end in a fine point ; 

 in transverse section they are semicircular, and may be seen to bo 

 made uj) of a fii*m structureless substance, in which no distinct cells 

 can be made out in the adult. The cindermis is formed by a single 

 layer of thin flattened cells. 



The glandular cells were only observed in one species, Spadella 

 cephaloplcra. Lamellar in form, and from three to five in number, 

 they are arranged in rosette shape around a central point, so that 

 they seem to be organs of attachment. They give a " warty " appear- 

 ance to the ventral surface, on which alone they are developed ; thuy 

 are most common near the tail, and become rarer and smaller ns we 

 approach the head. The constituent cells are cither cubical or 

 cylindrical in form. 



The sensor 1/ organs are cither tactile, optic, or olfactory, but in all 

 cases tlioy retain their relations to the ectoderm. The first of thcso 

 form small elevations, provided with still tactile setno ; they may be 



