56 



LUCERlSfARI^ Ai\l) THEIR ALLIES. 



incr 



_.._ -cases to its maximum depth, wliich is, on the average, fully, if not more than, 

 one-half that of the chondromyoplax (h). It is rather irregular and undulating on 

 its inner free surface ; and on its other face it varies exceedingly, according to the 

 irregularities of the chondromyoplax ; tilling every conical pit and diverticulum of 

 the latter as if it had been moulded upon it. The chief peculiarity of this layer 

 rests in the histological elements, and to that section of the subject the reader must 

 therefore turn for further information in regard to the more intimate structure of 

 the organ. 



§ U. The Caudal A,llir.rent Dish (Jh/s. 17, 18, 10, 37, 46, 51, y). 



114. The place which this part of the body holds among the organs is not so 

 much due to its being a distinct anatomical form, as to the fact that it is the basis 

 of certain functions which it performs in common with the anchors. Still its con- 

 struction is such that it is evidently something more than a mere caudal terminus. 

 Wliether we look at it as a means constructed and adapted to a certain end, or as 

 a means wliich has found itself best fitted for the purpose to which it is now put, it 

 matters not ; creative ingenuity would appear just as potential in the one case as 

 in the other. The simplicity and slight ditferentiation which the organ — if we 

 may so call it — presents, renders it none the less capable of doing what it, ostensibly, 

 is intended to do tlian if it were the most highly complicated of all the systems of 

 tlie body. Differentiation does not necessarily tend to the better performance of 

 any one duty, but to the separation of two or more functions which may have been 

 confided to the care of some organically simple organ. Every physiologist knows 

 that, although the nutritive system of the higlily complicated vertebrate animal is 

 subdivided into an intestinal tract, to break down the food, etc. etc., and a set of 

 bloodvessels to carry the absorbed nourishment to the tissues, these tissues are no 

 more faithfully and thoroughly su])plied with nourishment than those of the simple 

 Polyp whose general cavity is botli a digestive system to prepare the food for 

 absorption and a circulatory system to bring the assimilable matter into contact 

 with the tissues. 



115. A very slight modification of the form of the caudal shaft suffices to give 

 it the appearance of an official character, although we can readily conceive that it 

 could perform its present duty without its discal expansion. The comparative 

 extent of adherent surface would be the only element concerned. A change from 

 a truncate terminus to a rounded or pointed one would make a great deal of differ- 

 ence as to the nature of the substance adhered to, whether flat or irregular. This 

 species always clings to flat surfaces ; and, as it lives where the currents and wave 

 action are very strong, its expanded disciform tail seems to be eminently adapted 

 to increase its power of resisting the tractivt; force of the moving water. The 

 principal modifications of the layers of the shaft in the formation of the adherent 

 disk consist, in the first place, in a rather abrupt expansion of the whole, followed 

 nnmcdiately by a sudden changing of the course of the outline to a direction at 

 right angles to its previous trend ; virtually truncating the caudal terminus. Tlie 

 corresponding internal changes are noticeable chiefly in the ectophragma {Jig. 19,./'') 



