216 



the performance of the function of reproduction or egestion. This 

 appears to be precisely what occurs in the hydroids and polyzoa. In 

 the hydroids the egestive processes ave not properly known, but the 

 gonocytic function is conducted by the loss to the colony of one of its 

 individuals (the medusa) which carries with it the gonocytes which in 

 most cases wander into it, before departure, from various parts of the 

 colonial individual. 



The colonial Pedicellina apparently throws off intermittently 

 polypoid cups, charged with detritus, and this has already been con- 

 strued into an excretory process. 



Again , in the Ectoprocta , there is a periodic loss the to colony 

 of one of its individuals with the accompanying formation of a »brown 

 body«. Harmer^^, O s tro urn off ^6 and other observers agree in re- 

 garding this formation of a »brown body« as in part, at least, an ex- 

 cretory process. The latter definitely connects it with the absence of 

 nephridia, and if the nephrostome has an egestive function there is every 

 reason to suppose that the loss of the nephrostome would involve the 

 formation of a mass of egestive detritus. There is no reason why an 

 excretory function should not be united in the »brown body« with that of 

 egestion. In the allied function of reproduction, in some forms (Flustra, 

 etc.), the ova are found to occupy the whole zooecium, and presumably 

 this involves the loss of an individual of the colony. 



4) One interesting case may lastly, be cited, namely, that of the 

 Hirudinea. In these animals the nephridial funnels although still pre- 

 sent, do not open into a spacious coelom : the excretory part of the 

 nephridium being, however, well developed. In them also, we find 

 that their very food and mode of life, as in truly parasitic animals, 

 precludes all necessity for monocytic ingestion 4'', and hence indirectly 

 for monocytic egestion. 



5) This case is closely allied to that of the endo-parasitic animals. 

 In these, the food is in such a condition as to preclude the necessity 

 for all ingestive processes, either monocytic or polycytic, and is ready 

 for absorption into the tissues. Hence there is no necessity for either 

 polycytic anus or for nephrostomes, and although, excretory organs 

 are abundant, yet nephrostomes are invariably absent 4^. 



45 Quart. Journ. Mie. Jouru. XXXIII. 



46 Arch. Slaves de Bish. t. II. 1886. 



^'^ Compare also A. Sedgwick Q. J. M. S. XXVIII. »It is interesting to note 

 the resemblance which would exist between the transverse section of Peripatus and 

 the transverse section of a Leech, if the blood tracts of the former were more broken 

 up and the nephridia of the latter did not open internally into the vascular system« 

 and p. 20 preceding. See also A. E. Shipley, Stud. Morph. Lab. Camb. 1890. 



48 Flame-cell organs are, however, very common, confirming the purely ex- 

 cretory function of these structures. 



