i88 



CHAETOGNATHA 



or brain situated in the head, and of a ventral ganglion lying 

 in the trunk ; both these nerve centres are embedded in the 

 epidermis, and are connected with one another by means of two 

 stout peri-oesophageal nerves (Figs. 102, 104). The brain- 

 also gives off a pair of nerves to the eyes, another pair to the 

 olfactory organ, and a pair which ultimately meet one another 

 and so form a ring ; on this are 

 certain ganglia giving off nerves which 

 supply the muscles of the head. Both 

 the chief ganglia give off numerous 

 nerves, which divide and split up into 

 a network of fibres which permeate 

 the whole skin. 



The sense organs are comparatively 

 simple. A pair of very small eyes lie 

 in the skin of the head ; they are of 

 complex structure, and to some extent 

 remind one of the simple eyes of 

 certain Crustacea. Behind the eyes 

 and also on the upper surface of the 

 animal is an unpaired organ which is 

 usually described as olfactory in func- 

 tion (Figs. 103, 105). This is a 



FIG. 102. KendofSagittabipunc- ring- shaped modification of the epi- 

 tata. A, Dorsal view ; B, veu- dermis drawn out into different shapes 



tralyiew. x about 33. From . . -mi j-Z j 



Hertwig.) A, a, spines; b, m the various species. The modified 

 nerves to lateral cephalic gan- epidermal cells bear long cilia. The 



gha ; c, hooks ; d, cephalic r 



ganglion ; e, olfactory nerve;/, remaining sensory organs found in the 



j, olfactory organ : B, a, c, cells scattered in round groups over 



and g as in A ; ft, mouth. the ^^^ of the body and Q f the fins 



The central cells of each group bear long tactile hairs, and are 

 surrounded by supporting cells. 



The Chaetognatha are hermaphrodite, and carry the female 

 organs in the trunk, the male in the tail. In a mature specimen 

 the two ovaries occupy almost all the space in the trunk 

 between the alimentary canal and the skin, and each is supported 

 by a narrow lateral mesentery. The ovary is traversed by a 

 oviduct which often contains spermatozoa ; it is not clear how 

 the eggs make their way into the oviduct, which seems to have 



