192 THE ACEPHALA. <§<§ 175, 175 



in some species covers even tbe whole of the siphon,^ Very often, how- 

 ever, this epidermis is worn away upon old portions of the shell, which 

 is quite striking with those which have lamelliform or pilous prolongations 

 around the borders of the shell/'*> 



The two shells are joined together partly by a hinge [Car do), and partly 

 "by an elastic tissue [Ligamentum). ^^*^^ This last, either external or inter- 

 nal, is antagonistic to the adductor muscles of the shell. It is composed 

 of elastic fibres, the internal of which, when the shell is closed, are com- 

 pressed between the borders of the hinge, while those which are external 

 are lengthened out. In both cases, their natural action is to open the two 

 shells. '1^' 



§175. 



The Terelratidae have avery remarkable internal calcareous support situ- 

 ated upon the inner surface of the two shells. It consists, first of two delicate 

 outwardly curved peduncles, which arise from the sides of the two cardinal 

 teeth situated upon the non-perforated valve ; then there are two other pe- 

 duncles which are shorter, and arise from a longitudinal ridge upon the 

 centre of the same valve ; these pass in front and unite in an arcuate man- 

 ner. The two branches thus formed are abruptly recurved after a short 

 course, and unite, forming a common arc behind the centre of the shell. ^'^ 

 With many, this structure Is much more simple, consisting only of a median 

 apophysis, from which pass off two alar prolongations which are curved at 

 their extremity. ^-^ This structure serves principally for the insertion of 

 the tentacles. '^^ 



CHAPTER II, 



MUSCULAR SYSTEM AND ORGANS OF LOCOMOTION. 



§176. 



The muscles of the Acephala are composed of simple, smooth fibres. 



8 With Mya, and Lutraria, the epiJermis forms reous formation which constitutes, with the tw« 



a complete sheath around the siphon. united valves, the singular tul)e of these animals. 



y Mytilus hirsutus, Area barbata, lacerata, " The ligament is internal with Pec ten, Span- 



and ouala. dylus, Mya, Lutraria, and P/io/as ; but external 



10 I must omit a description of the various forms with the Cliamacea, Cardiacea, Arcacea, and Nai- 



of the hinge and ligament, for they belong properly ades. It is half external and half internal with 



to the department of Zoology. The hinge is wholly Malleus, and many other species, 



wanting with the Inclusa, and the ligament is ab- 1 Terebratula chilensis, dorsata,dentata,a,üi 



Bent with the Brachiopoda ; and with Orbicula, Soiverbyi ; see Owen, loo. cit. PI. I. fig. 4. 



and Lini^ulu, both are aljsent. 2 Terebratula rnbirunda, wnApsittacea. 



The Aspergillina are distinguished from all the 3 According to Owen, in those species which 



other Acei)liaia, by a singular disposition of their have this ajjparatus highly developed and bent 



valves. Their mantle ceases early to secrete the backwards, these arches, notwithstanding their cal- 



matter for the shell-formation. The two valves are careous nature, are somewhat elastic ; and wlicn 



then joined at a point, where, most probably, they the valves are closed, they are slightly depressed, 



would have been articulated, while the mantle and thus may serve in the absence of the elastic 



which lias only two small openings, and its long ligament, for the opening of the shell. 

 ViphoQ with a. double canal, is covered with a calca- 



