570 THE ANATOMY OF INVERTEBRATED ANIMALS. 



may be its characteristic features, shades off at its margins 

 into some other group ; and the object of classification is 

 simply to bring into prominence the morphological types 

 which embody these characteristic features. 



It appears to me impossible to compare the structure and 

 the larval conditions of a Polyzoon with those of a Brachio- 

 pod, without arriving at the conclusion that they are more 

 closely allied with one another than they are with any third 

 group. Nevertheless, the Polyzoa approach the Hotifera, 

 and the Prachiopoda the Annelida, on the one side ; while 

 on the other they present unmistakable affinities with the low- 

 er Mollusca. At the same time the weight of the resemblances 

 between the Polyzoa and the Tunicata, which led Milne- 

 Edwards to the establishment of the group of " Mollusco'ides " 

 (adopted by myself under the title of Molluscoida), has been 

 much lessened by the progress of investigation. 



I conceive that we may best keep these resemblances and 

 differences in view by associating the Polyzoa and the Pra- 

 chiopoda into a division apart, for which I propose the name 

 of Malacoscolices ; in order to indicate its relations with 

 the Worms on the one side and with the 3Iollusca on the 

 other. 



The Tunicata are absolutely distinguished from all other 

 iuvertebrated animals except Palanoglossus, by the perfora- 

 tion of the pharynx and its conversion into a respiratory 

 organ. 1 



At first sight there appears to be little ground for the 

 approximation of groups apparently so widely different as the 

 Tunicata and the Enter opneusta. But the extraordinary 

 similarity in the structure of the perforated pharyngeal sac in 

 the larvae of Tunicates and of Palanocjlossus is a fact of 

 great morphological weight. An ecaudate Appendicularia 

 of those species which have the alimentary canal nearly 

 straight, would be marvelously like a larval Palanoylossus, 

 which is again little more than a specially modified Turbella- 

 rian. I think, therefore, that the Tunicata and the Entero- 

 pneusta may properly constitute a division of Piiaryngo- 

 pneusta. 



1 I have alluded above to the structures described by Semper in some Oli- 

 gochceta and in Sabella. I do not doubt the accuracy of the description ; but 

 it does not lead me to conclude that the structures in question are homologous 

 with either Vertebrate, Enteropneustal, or Tunicate branehiio. 



