162 CTENOPHOR.E. Part II. 



bilateral symnietiy of the Ctenoplioraj even more conspicuous, and certainly rniore 

 perfect, than in most Echinoderms, notwithstanding the presence of an odd zone 

 in these ; for as soon as the anterior extremity of tlie body assumes prominence 

 by its specialization, the sides recede, as it were, to a lower relation. But in 

 Ecliinodcrms there is no such specialization of the anterior extremity: the structural 

 progress of this class over the Acalephs consists only in the introduction of an 

 odd zone. This zone is mostly identical in structure with the lateral zones, and 

 scarcely ever so far differentiated as to give preponderance to the sides in the 

 general configuration of the body. In Ctenophorte, on the contrary, the aljsence 

 of an odd spheromere, combined with the identity of development of tlie anterior 

 and of the posterior pairs, and the differentiation of the two lateral pairs, including 

 an additional, interambulacral chymiferous tube, throws the whole weight of the 

 extreme structural differences of the sjoheromeres upon the sides, in such perfect 

 balance with reference to tlie antero-posterior diameter, tliat the reciprocal action 

 of the most imi^ortant function in the life of these animals is to be traced in 

 the alternate contractions of the two sides of the )jody. Again, tlie opposite poles 

 of the main axis are strikingly contrasted : at one end we find the mouth or 

 actinostome, and at the other end the circumscribed area, and in an asymetrical 

 relation to it, the two discharging openings of the chymiferous system. The 

 curves of the sides contribute also to render the contrast between the actinal and 

 abactinal j^^les more prominent. Thus, in Ctenophorte the opposite ends of their 

 three diameters are evenly balanced, presenting identical parts in antitropic relations 

 on the anterior and jjosterior sides and on the lateral sides of the body, and 

 heterogeneous parts in similar relations on ojjposite jioles. 



As in all Radiates, the body of the Ctenophoraj is a spherosome ; that is to 

 say, it is essentially radiated in its structure, and as in all Acalephs it consists 

 of cells, and of cells only, variously combined and of a variety of forms. There 

 are no specialized tissues in it. The distinction of a muscular system, as described 

 in my former papers upon Acalephs, was a mistake, as will be shown hereafter, 

 arising from the peculiar constitution of the motory cells ; nor is there a distinct 

 nervous system. The Avhole bulk of the body is made up of large contractile cells, 

 and is covered with epithelial cells, which also line the digestive cavity and the 

 system of chymiferous tubes arising from its abactinal prolongation. The thickness 

 and form of the spherosome vary in different families ; the size and form of the 

 digestive cavity, and the mode of ramification of the system of chymiferous tubes, 

 also exhibit striking differences : but all Ctenophoraj agree in this, that the sphero- 

 some has a uniform structure, being made up of a continuous mass of large motory 

 cells, combined into distinct systems, bearing definite relations to the various and 

 complicated motions of the different parts of the body. The assumption of Mertens, 



