THE CHJETOGNATHA. 541 



only member of this group, comprises several species of small 

 animals which are found swimming at the surface of the ocean 

 in all parts of the world. Although the whole structure and 

 course of development of Sagitta are now very well known, 

 its true atlinities are not definitely settled. Anatomically, it 

 approaches the Nematoid worms and the oligochsetous Anne- 

 lids in some respects ; but its development presents pecul- 

 iarities which are as yet unknown among these animals, while 

 they occur among the BracMopoda and the JEchinodermata. 

 The body of Sagitta (Fig. 154), rarely more than an inch 

 long, is elongated, subcylindrical, and unsegmented ; it is en- 

 larged at one end into a rounded head, while at the other it 

 tapers to a point. There are no parapodial appendages, but 

 the chitinous cuticle is produced into a finely-striated lateral 

 fin on each side of the body and tail, and into delicate setae. 

 On each side of the head there are a number of strong, curved, 

 claw-like, chitinous processes, which can be laterally divari- 

 cated and approximated, and serve as jaws. Between them 

 is the mouth ; and at the sides of the mouth are four sets of 

 short but strong spines. The mouth leads into a simple and 

 straight intestine, which opens by an anus situated on the 

 ventral face of the body, where the tapering caudal region 

 commences. A dorsal and a ventral mesenteric band connect 

 the intestine with the wall of the body, and divide the peri- 

 visceral cavity into two chambers. Beneath the ectoderm 

 lies a layer of longitudinal, striated, muscular fibres. The 

 nervous system consists of a large oval ganglion, which lies 

 in the middle of the ventral wall of the body, and sends off 

 anteriorly two commissural cords, which unite ^\ith a supra- 

 oesophageal ganglion. Among other branches, this gives off 

 two to the dorsal side of the head ; these dilate at their ex- 

 tremities into spheroidal ganglia on which the ej^es rest. The 

 ovaries are elongated tubular organs, which lie one on each 

 side of the intestine, attached to the parietes of the body. 

 Their ciliated ducts open close to the vent and are provided 

 with dilatations which serve as receptacula seminis. Behind 

 the anus the mesenteric lamina3 unite and form a vertical 

 partition, which divides the cavity of the caudal part of the 

 body into two chambers. On the lateral walls of these, cellu- 

 lar masses are developed, which become detached, and, float- 

 ing freely in the perivisceral fluid, are developed into sper- 

 matozoa. The latter escape by spout-like lateral ducts, the 

 dilated bases of which may be regarded as vesicul^e semi- 

 nales. 



