412 



THE CIRRIPEDS, TUNICATES AND ECHINODERMS. 



to the exterior through the nephridia-like ducts of the shell gland, mx.d. They 

 probably represent the combined thoracic cceloms together with the excretory 

 portion of the "shell gland" or "coxal gland" of the second pair of maxillae. 

 The third pair, 6 3 , lie external to and somewhat behind the second. They are 

 completely closed, and probably represent the remnants of several pairs of united 

 abdominal ccelomic chambers. 



Excretory Organs. Two pairs of nephridia-like excretory organs are 

 conspicuous in the arthropods and remarkably constant in their location. The 

 so-called green gland of the second antennae, and the shell gland of the second 

 maxillae of Crustacea (coxal gland of the fifth pair of thoracic appendages in 

 arachnids). The characteristic thin-walled end sac of these organs is derived 

 from a portion of the ccelom, but a variable one. It may represent a single 



Re. 



Ms.c. 



?r.C. 



Br.C. 



c. 



FIG. 279. Diagrams to illustrate the relations of the coelomic chambers, and their excretory ducts, to the 

 main subdivisions of the body (tagmata) and to the central nervous system. A, Larval copepod (Lernsea); B, an 

 adult cirriped; C, Balanoglossus. 



somite (Limulus) or a small part of one (Lernasa), or the whole, or a part of several 

 combined somites (cirripeds and other acraniates). 



In the cirripeds the excretory organs of the fifth metamere have very volum- 

 inous end sacs, c 2 , and the nephridia-like tubes that lead off from them open at 

 the base of the second pair of maxillae. A small pair of excretory sacs lie in the 

 head region in front of the mouth that probably represent the remnants of the 

 antennary glands of other Crustacea. 



The sexual organs are of exceptional volume in the cirripeds, the racemose 

 testis ramifying through the whole trunk and opening at the apex of the modified 

 tail lobe. (Fig. 275.) The location of the ovaries is noteworthy in that they 

 lie mainly in the cephalic stalk and anterior portion of the mantle, the ovarian 

 lobules, in some cases, extending into the recesses of the exoskeleton. The 

 oviduct opens outward near the middle of the body, at the base of the anterior 

 pair of abdominal appendages, o.d. 



