194 



THE ACEPHALA. 



»§.178. 



As the points of the insertion of these muscles do not always correspond 

 with the two valves/^^ their direction is often oblique, or, they sometimes 

 ■even cross each other.® Orbicula and Lingula, which want both the liga- 

 ment and hinge, have this disposition of the adductors which terminate 

 usually by a delicate tendon, and the contraction of either of these muscles 

 alone, produces the lateral movements of their valves. 



The spirally -pointed tentacles of the Brachiopoda are moved by a par- 

 ticular apparatus. The fringes of these organs are inserted upon a carti- 

 laginous tubular prolongation which tapers to a point. This is closed at 

 both extremities and contains a liquid, which, by the contractions of the 

 circular muscular fibres, is propelled from the base to the extremity, thereby 

 unrolling the spiral turns. '^* These tentacles here certainly take the place 

 of the elastic ligament of the bivalves, for their extension probably tends 

 to slightly open the valves. 



With the Lamellibranchia, the tentacles which are not rolled, are ar- 

 ranged quite differently. Like their branchiae, they are scarcely at all 

 irritable, — this being due to the fewness of their muscular fibres. But in 

 the mantle these fibres are very abundant, and especially near their free 

 borders. This is true also of the Siphon, in which both longitudinal and 

 circular fibres can be easily seen ; here, two very distinct flattened muscles 

 arise from the base of the siphon and are inserted upon the two valves, ex- 

 ternal to the posterior adductor muscle : these serve as a Retractor siphonis. 



§ 178. 



Very many of the Lamellibranchia have a highly-developed organ of 

 locomotion, — i]xQfootß^ This is a muscular prolongation from the ventral 

 surface, which passes obliquely forward to be inserted upon the internal sur- 

 face of the back of the shell, by four, rarely more, tendinous cords. ^-^ 



These cords surround the abdominal viscera, and becoming gradually 

 thicker and more muscular, finally blend with numerous, interlaced muscu- 

 lar fasciculi which compose the foot. This last varies considerably as to its 

 size and form, and can be protruded a long way out through the open shells, 

 but may also be wholly withdrawn.''^ 



2 Lingula forms an exception. Here the pi'in- 

 ■cipal adductor is a short, solid muscle, which 

 stretches straight across from one shell to the 

 other, at their posterior extremity. 



3 This muscular ai>paratus has been particularly 

 •described and figured by Owen, and Vogt (loc. 

 cit.) with Terebratu/a, Orbicula, and Lingula. 



With the Terebrntula, two pairs of muscles 

 arise from each valve. The two anterior ones 

 arising from the imperforate valve, are the longer, 

 and have their origin back of its centre. After the 

 •crossing of their delicate tendons, they pass 

 through the opening of the valve and terminate in 

 the jjeJunclo, together with the two posterior ones 

 which are short and fleshy, and which arise at the 

 base of the hinge. Of those of the perforated 

 valve, the two posterior ones only pass to the 

 peduncle — the two anterior being attached to the 

 baae of the other valve. With Orbicula, there 

 are two posterior, and two anterior fleshy muscles, 

 all of which pass obliquely from one valve to 

 the other, while the anterior ones sometimes send 

 oö" fibres to the short j)eduncle. In the si)ace cir- 

 cumscribed by these four muscles, lie four others, 

 which are small and interlaced, and extend from the 



visceral sac to the valves. With Lingula, there 

 are, beside the principal adductor, four pairs of in- 

 terlaced muscles, which pass obliquely through the 

 centre of the cavity of the valves, and are attached 

 by their two ends to the visceral sac. 



4 Owen, loc. cit. ; and ^ogt, Anat. d. Lingula, 

 p. 8, Tab. II. fig. 16-18. 



1 The foot is absent, particularly with those mol- 

 lusks which are fixed to rocks and other solid 

 bodies, by a calcareous cement. 



2 Usually one pair of these delicate cords passes 

 above, and another below, and are inserted upon 

 the valves, quite near the four points of insertion 

 of the two adductor muscles. This is so with Ano- 

 donta, Unio, Cardium, &c. With Isocariiia, I 

 have found a thu-d pair of cords inserted upon the 

 j)osterior extremity of the summits of the shell. 

 These serve not only as Retractores of the foot, 

 but when this last is fixed to some point, draw the 

 animal towards it. 



3 The laterally-compressed foot of >4wodon<a, and 

 Unio, arises, by a large base, from the abdomen, 

 and has carinated borders. That of Pectuncit/u« 

 and Venus, is quite similar, but its free border is 

 hollowed by a furrow, and is therefore bi-carinated. 



