THE CRUSTACEA. 53 



cords. The anterior cords unite in front of and above the 

 mouth, into a hexagonal ganglion. This gives off two branches 

 which dilate at their extremities into the spheroidal ganglia, on 

 which the darkly pigmeuted imperfect eyes rest. The ovaries, 

 saccular organs, lie on each side of the intestine and open on 

 either side of the vent ; receptaeula seminis are present. Be- 

 hind the anus, the cavity of the tapering caudal part of the 

 body is partitioned into two compartments ; on the lateral 

 parietes of these, cellular masses are developed which become 

 detached, and floating freely in the compartment, develope 

 into spermatozoa. These escape by spout-like lateral ducts, 

 the dilated bases of which perform the part of vesiculse semin- 

 ales. The embryos are not ciliated, and undergo no meta- 

 morphosis. 



XX. THE CRUSTACEA. 



In this class (Fig. 27), the body is distinguishable into a 

 variable number of " somites," or definite segments, each of 

 which may be, and some of which always are, provided with a 

 single pair of articulated appendages. The latter proposition is 

 true of all existing Crustacea : whether it also held good of the 

 long extinct Trilobites, is a question which we have no means of 

 deciding. In most Crustacea, and, probably in all, one or more 

 pairs of appendages are so modified as to subserve manducation. 

 A pair of ganglia is primitively developed in each somite, and 

 the gullet passes between two successive pairs of ganglia, as in 

 the Annelida. 



No trace of a water-vascular system, nor of any vascular 

 system similar to that of the Annelida, is to be found in any 

 Crustacean. All Crustacea which possess definite respiratory 

 organs have branchiae, or outward processes of the wall of the 

 body, adapted for respiring air by means of water ; the terres- 

 trial Isopoda, some of which exhibit a curious rudimentary 

 representation of a tracheal system, forming no real exception 

 to this rule. When they are provided with a circulatory organ, 

 it is situated on the opposite side of the alimentary canal 

 to the principal chain of ganglia of the nervous system ; and 



