OF CONCHOLOGY. 43 



has been pointed out by Morse and others, may be regarded as 

 the typical group of the sub-kingdom. The pteropods present no 

 features of class value, much less the heteropods. 



The fact that the pulmonates have been separated as a distinct 

 class by Huxley* excites surprise, as the characters given are 

 insufficient for such a distinction, and very gradual transitions 

 toward the other gasteropods may be pointed out. The flexure 

 of the intestine varies greatly, and is by no means a constant 

 character. 



A transition between the lamellibranchs and tunicates is in- 

 dicated through the genus Rhodosoma (Schizascas, Stimpson) and 

 Prof. Morse observes f " the relations between the tunicates and 

 lamellibranchs are too obvious to indicate." It has been pro- 

 posed by Grirard to combine the Planarian worms with the mol- 

 lusks on embryological and anatomical grounds ; and by Moerch 

 that the trematodes, cestodes and other low forms of worms should 

 also be transferred to this sub-kingdom, but their obvious affini- 

 ties, in spite of their simplicity of structure, have deterred other 

 naturalists from following these examples. 



In some points of their structure, the brachiopods are allied to 

 the Conchifera, and by others to the cephalopods and some aber- 

 rant groups of gasteropods. So intercrossed are the threads of 

 affinity, that a kind of reticulation is the result, and it is far 

 more easy to point out these features than to assert for some a 

 preponderance over all others. 



The heart (though far more simple) of the brachiopods with its 

 accessory '-hearts," or "pulsatile vesicles" and their relations 

 to the blood vessels which supply the branchial apparatus, is 

 closely paralleled both in appearance and in function, in the so- 

 called " branchial hearts" of the cephalopods. £ In the conchifers 

 also a notable dilatation of the blood vessels§ on their entry into 

 the branchiae may be observed, though not differentiated as 

 strongly as in the others. || The heart lies free upon the intestine 

 in Anomia^, much as it does in Terebratula, and the greatly en- 

 larged oral "tentacles," the absence of other branchiae, or of a 

 large, well differentiated "foot," as well as the complex muscu- 

 lar system and the peduncular attachment, all recall a somewhat 

 similar state of things which exists in the brachiopods. In some 



* Introduction to the Classification of Animals, p. 39, 1869. 

 t Class. Moll, on the Basis of Cephalization, Cora. Essex Inst. p. 176, 

 1865. 



t See Bronn, Kl. and Ordn. Thierr. 2d Abth. Taf. cxvii. 



\ The circulating vessels are most simple in the aberrant Solenoconchce. 



|| Garner, Tr. Zool. Soc. Lond., ii, p. 90, 1841. 



II Ibid. 1. c.p. 91. 



