ON THE BRITISH ANNELIDA. 163 



mata no true blood exists, defining the blood as a fluid circulating in a di- 

 stinctly and independently organized system of vessels. In the Zoophytes 

 and Acalephae, the blood is replaced by a fluid, the basis of which is sea- 

 water. In the organism of the Echinoderm and the Annelid, notwithstanding 

 the superaddition of a distinct system of vessels for the blood proper, this 

 fluid, the characters of which will be afterwards described, constitutes an or- 

 ganic zoological character more significant of locality in the series than any 

 other single element of structure. Guided by the affinities of this important 

 fluid, the naturalist may discover unquestionable points of serial approxima- 

 tion between the Annelid and Echinoderm through the Sipunculidce on one 

 side ; at another angle of the group, the Aphrodita aculeata exhibits the most 

 striking resemblance in structure (the mere outward form excepted) to the 

 AsteridcB. 



In another direction the Annelida approach the inferior Mollusca. Many 

 features of analogy exist between the Tunicata and the Echinodermata 

 through the genus Pelonaia, which in its general form and in some structural 

 characters betrays an obvious tendency towards the Sipunculidce, whilst in 

 other respects in that group an undoubted approximation is traceable to the 

 annulose division, in the appearance of a disposition to bilateral symmetry in 

 the packing of the viscera and the transverse segmentation of the external 

 tunic. Among the Gasteropod Molluscs, as originally indicated by Milne- 

 Edwards*, an approach is made through the Chitons in the direction of the 

 annulose families. It was M. de Blainville, however, who first suspected this 

 alliance, and accordingly constituted a special group, transitional between the 

 Mollusca and Annelida, under the name oi Polyplaxiphora, In the transverse 

 segmentation of the shell of the Chitons, a remarkable resemblance is offered 

 to the covering of an articulated animal : nor is this resemblance limited to the 

 shell, for its several parts arc connected together by means of a complex 

 muscular apparatus, which enables the animal to move them one upon the 

 other in such a manner as to roll itself up into a ball : nor are there wanting 

 other similarities of external structure ; the generative apparatus presents a 

 symmetrical arrangement after the type rather of that of the Annelid than 

 of that of the Mollusc. The heart occurs under the character of a pulsatile 

 dorsal vessel, indicative of another affinity to the Annulosa. In the Chito- 

 nellus the tendency to the transverse division of the body is still more signi- 

 ficantly marked. Here the shell is not sufficiently developed to cover the 

 dorsum of the animal, yet its several rudimentary pieces are disposed at 

 regular intervals like the scales of the Aphrodite ; the vermiform elongation 

 of the body is more decided, and the circulating system is still more obviously 

 constructed on the plan of that of the Annelid. The respiratory and digest- 

 ive organs, and the lingual apparatus on the other hand, remind us, as sug- 

 gested by Prof. Forbes, of the corresponding organs of the Prosobranchiate 

 Molluscs, while the creeping disc is that of a true Gasteropod. 



It is a subject of surprise that the Dentaliadce, the molluscan nature of 

 which is now so conclusively established, should have received, in the last 

 French edition of the 'Regne Animalf,' a place among the tubicolous Anne- 

 lida. The researches of Deshayes and Savigny, and more satisfactorily those 

 of M. de Blainville and Mr. Clark J, have abundantly proved that these 



* Mem. on the Classification of the Gasteropoda, in Ann. des Sci. Nat. Ser. iii. Zool. 

 vol. ix. p. 102. 



f This is by far the most perfect and elaborate edition of this great work ever published, 

 the editorship of which has been conducted by the disciples of Cuvier. It is known as Cro- 

 chard's Edition. 



X See Annals of Natural History, Nov. 1849. 



m3 



