594 K. W. MAOBRIDE. 



tlie common ancestor of Coelomate animals was like. I think 

 it will be 'admitted that to call this ancestor a Ctenophore 

 would be extremely ill advised, as this would suggest that 

 in details of the arrangement of cilia, etc., it agreed with 

 the Ctenophora. Better, I think, to adopt the name already- 

 suggested by me in my paper on Asterina gibbosa (18), 

 viz. Protocoelomata, for the ancestral group from which 

 Ctenophora, Platyhelminthes, Annelida, Mollusca, and Echi- 

 nodermata have sprung. To this group Vertebrata are also 

 to be traced back, since the Tornaria larva of Balanoglossus 

 exhibits the three-fold division of the coelom and the apical 

 nervous plate, which we have determined as characteristic of 

 the ancestor of Echinodermata, whilst the middle division of 

 the coelom in the allied form Oephalodiscus is produced into 

 long tentacles, just as is the case with the hydroccclc in 

 Echinodermata. 



Turning now to the later development of Ophiothrix 

 fragilis, the principal point to be noticed is that the ontogeny 

 bears out the conclusion of comparative anatoni}' that the 

 Asteroidea and Ophiuroidea are closely allied. Thus in the 

 formation of one perihaemal rudiment from the axial sinus, in 

 the apposition of arm-rudiment 1 and hydrococle-lobe 2, in 

 the opening of the stone-canal between lobes 1 and 2, in the 

 development of dorsal and ventral horns from the left posterior 

 ccelom, and finally in the development of the peri-oral coelom, 

 the development of Ophiothrix recalls that of the Asterid. 

 The points in which it differs are, first, that the fixed stage is 

 entirely missed out; second, that the cavities are in general 

 smaller and their walls thicker; third, in the development of 

 epincural folds; and fourth, in the persistence of the larval 

 mouth. Taking these points in order, the missing out of the 

 fixed stage is in accordance with the rule everywhere exempli- 

 fied in the animal kingdom, that there is a tendency to reduce 

 the number of larval stages. In each stage the organism is 

 exposed to a special set of dangers; if, therefore, enough food 

 can be accumulated during one stage to enable the necessary 

 changes in structure to be made, so that the next stage can 



