

AFFINITIES. 161 



sacs they present a certain resemblance to the Chordata. In 

 all the members of that great group with the exception of the 

 Tunicata, the coelom in its first state in the embryo presents 

 traces more or less marked of three divisions : these are (1) the 

 anterior or proboscis coelom, which in Vertebrata and Entero- 

 pneusta is single, in Amphioxus double, (2) the collar or middle 

 coelom which is always double, and (3) the trunk coelom which 

 is double and which in Vertebrata and Amphioxus becomes 

 met americ ally segmented. In Echinodermata we seem to be 

 able to make out indications at least of a similar tripartite 

 division. We have (1) the anterior coelom which is sometimes 

 single (Asterina), sometimes double (Echinus], (2) the hydrocoel 

 which is probably fundamentally double though in some cases 

 only one hydrocoel sac is formed (Holothurians, Crinoids), and 

 (3) the posterior coelom which is always paired. Of these the 

 hydrocoel presents the peculiarity of growing out into ten- 

 tacles a feature which is also presented by the middle division 

 of the coelom in the enteropneust genera, Cephalodiscus and 

 Rhabdo pleura. But with these similarities we have to note 

 certain differences. In the first place in the Chordata, in which 

 the enterocoelic origin of the coelom is clearly presented, these 

 three divisions of it always come off from the enteron separately, 

 whereas in Echinoderms the enteron at most gives off only one 

 pair of coelomic sacs. Further, whereas in the Chordata the 

 middle coelom (collar) is never associated more closely with the 

 anterior than with the posterior, in the Echinoderms it is always 

 closely associated with the anterior coelom, being developed 

 from it and remaining connected with it by the stone-canal 

 throughout life. With regard to these differences we have only 

 to say this : that they are differences such as we might expect 

 from the greater remoteness of the Echinoderms from the Chor- 

 data than of any of the Chordata from each other, but that they 

 are not sufficiently great to put out of court the homologies 

 suggested by the comparison. 



To turn to other points of resemblance : we have the resem- 

 blance (1) in the central nervous system, (2) in the skeletal 

 system, (3) in the shifting of the mouth and in the asymmetry 

 of the body, and (4) in the larval form. To take these in order : 

 (1) In the Chordata, as is well known, the central nervous system 



z in M 



