38 C'OMl'AliATlVE ANATOMY. 



of complexity. In thfe young of some of the Neomeniiiia these last named 

 canals are simple tulies of about e(iual calibre throughout, but in later life 

 they become modified into a shell gland ami one or more seminal receptacles. 

 In adult Chaetodermatina these canals remain comparatively simj^le, and there 

 is some evidence that they function as excretory organs as well as genital ducts. 



In every case the coelomoducts originate from the hintler wall of the peri- 

 cardium as comparatively small, ciliated tubes which pass anteriorly to about 

 the level of the forward wall of the j^ericardium where they make a shai'p licnd 

 and join the so-called shell gland in the Neomeniina. At the intersection of these 

 two divisions from one to twenty-five vesicular appendages are usually attached, 

 which have usually been considered seminal receptacles though the arrangement of 

 the sperms in a few species indicate that for at least a part of the breeding season 

 they may function as seminal vesicles. As the distal or ventral portions of the 

 coelomoducts usually unite before entering the cloaca the shell gland which they 

 form is a bicornuate, comparatively swollen structure with excessively thick walls 

 and contracted lumen. It is reasonably certain that this organ functions as a 

 nidamental gland, forming possibly an all)iunenous envelope before the egg 

 passes to the exterior. 



In intimate connection with the genital apparatus just described there are a 

 number of problematical organs which in some cases at least appear to have some 

 role to play in the reproductive process. These include the genital spicula or 

 penes such as occui- in Neomenia carinata, Donjmenia acuminata, Parnrhopalia 

 pruvoti, Stijlomcnia salvalori, and several other species. These are either rela- 

 tively large calcareous jiairetl spines, which ordinarily are concealed in sheaths 

 formed as anteri<n-ly directed diverticula of the cloacal wall, in'ovided with pro- 

 tractor and retractor muscles and in some cases with a gland, or similar diver- 

 ticula, may conceal numerous spines of much smaller size. While the function 

 of these organs is unknown it is reasonable to believe, with several authors, that 

 they are excitants and po.ssibly in a few species they may serve to attach the 

 animals in coitu. 



The preanal gland (Hubrecht), which is attached to the anterior face of the 

 cloacal wall in Proneomcnia shiiteri, and opens at the end of the pedal furrow or 

 right and left under the cloaca, may be associated in some way with the repro- 

 ductive process. 



Physiology. — Several authors, notably Wiren, have called attention to 

 the striking resemblance between the Chiton kidney ami the coelomoducts of 

 certain species of C'haetoderma; and again the presence of crystals has been 



