4 o 



CTENOPHORA. 



the Polyclads separate genital ducts, and even copulatory organs are found. These are, however, of a 

 very peculiar primitive type, as might be expected in forms where they are acquired as new organs. 



The nervous system affords some difficulties for the comparison on account of its doubtful 

 character in Ctenophores. It seems, however, beyond doubt that the cerebral ganglion of Polyclads 

 corresponds to the apical organ of Ctenophores, even if the latter is not directly to be regarded as 

 being a nervous apparatus. It develops as an ectodermal thickening in Polyclads and in a central 

 position; it is only later on in the development that it is separated from the ectoderm, contemporane- 

 ously moving forwards; in the more primitive forms, e.g. Planocera, it remains rather remote from 

 the anterior end of the body. According to Chun the eight main nerves of Polyclads are the homo- 

 loga of the eight ciliated ridges of Ctenophores. It is a consequence of the forward wandering of the 

 brain that the two posterior nerves become the main longitudinal nerves. The otolith, so highly cha- 

 racteristic of Ctenophores, has no homologue in Polyclads; in some Rhabdocoela there is, however, an 

 otolith above the cerebral ganglion. 



The histological structure shows rather conspicuous conformity. The epidermis has essentially 

 the same structure in both groups; to the very characteristic granular cells of Ctenophores the rhab- 

 dite cells of Polyclads are evidently homologous. On the other hand there is nothing in Polyclads 

 corresponding exactly to the colloblasts, which is easily understood from the fact that the tentacles, 

 when at all present, in Polyclads have completely lost the function as prehensile organs. The Poly- 

 clads are certainly ciliated over the whole surface, which is not the case in typical Ctenophores; but 

 here Coeloplana forms the connecting link, being ciliated as the Polyclads 1 . - The muscles are of 

 the same type in both groups, branched at the ends; that the musculature is considerably more 

 developed in Polyclads than in Ctenophores is only what should be expected from their different mode 

 of life. 



Concerning the movement of the Polyclads the highly interesting fact is pointed out that the 

 more primitive forms (Anonymus, Planocera) do not always proceed with the anterior end forwards; 

 they may also move sidewards in good accordance with their supposed origin from radiate ancestors. 

 With this also corresponds the fact that in the primitive forms eyes are found all round the margin 

 of the body. 



In the development there is conspicuous accordance in the cleavage and gastrula-formation 

 (epiboly); in both groups the gastrula-mouth develops into the definite mouth, the opposite pole 

 becoming the sensory pole. On the other hand the mesoderm appears to develop in a different way 

 in the two groups. In the larvae finally there is a peculiar feature, not without importance for the 

 comparison of the two groups, viz. that on the processes of the Polyclad-larvse the cilia are arranged 

 in transverse rows, the cilia of each row moving contemporaneously recalling the combs of the 

 Ctenophores. Lang hints at the possibility of homologizing the eight processes of the Polyclad-larvse 

 with the costse of Ctenophores, pointing out, however, as a main difficulty that they are not in the 

 same relative position to the axes of the body as the latter. 



With full right Lang states (Monogr. p. 665) that this hypothesis "scheint bei dem gegen- 

 wartigen Stand unserer morphologischen Kenntnisse diejenige zn sein, die den Ursprung der Bilaterien 



') According to the recent researches of Abbott Coeloplana is only ciliated on the ventral side. 



