^§ 179, 180. THE ACEPHALA. 195 



Most of these animals use this organ to dig in the sand, or to creep along 

 on soft surfaces. For this purpose, they reach it out in front, and then by 

 alternate contractions and elongations, drag their body after it. Some spe- 

 cies can in this way glide freely along like the Gasteropoda, or even seize 

 hold of aquatic plants.'*' Sometimes this foot is truncate and hollow at its 

 •extremity, and probably, therefore, acts like a sucker.'^' 



§ 179. 



With many of the Lamellibranchia,''^ the foot appears imperfectly de- 

 Teloped, and has a secretory organ of the Byssus, a part by which these 

 animals are attached to wood, stone, and other bodies. In this case the 

 foot is a delicate, protractile, tongue-like body,<^* capable of a stiflness suf- 

 ficient for creeping, but used chiefly as a feeler to find the points of attach- 

 ment by the byssus.*'^' It always points towards the oral extremity, and 

 upon its inferior surface there is a longitudinal furrow which has a cavity 

 at its base. The walls of this furrow and cavity secrete the byssus. From 

 their glandular aspect, they differ much from the rest of the organ, which is 

 formed of numerous interlaced muscular fibres.'** 



The bottom of this cavity from which the furrow arises, is regularly di- 

 vided by numerous delicate, parallel lamellae, from which arises the compact 

 root of the byssus, '''> This byssus is, therefore, inserted into the base of the 

 ■cavity as are our finger-nails into their matrix. Its base has a fibrous, or 

 lamellated structure, and passes into a longer or shorter trunk composed of 

 numerous cylindrical, or flattened filaments,'^' whose extremities are some- 

 times discoid,'''' 



§ 180. 



Many Bivalvia. which are likewise wanting in locomotive organs, and 

 tave, moreover, no organs of the byssus, attach themselves to bodies in 

 another and peculiar way. Thus, with Anomia, one of the valves is marked 

 by a deep fissure, across which, like a short peduncle, a portion of the ad- 



"With Tetlina, Donax^ and Cyc/as, it is very long 4 I do not yet clearly understand the true nature 



-and more or less ridged, and often quite small at of the walls which secrete the byssus. A. Müller 



its base. has designated them as Glandula byxsipara, com- 



With Cardium, Nucula, Trigonia, Madras posed of round cells. He athrms to have seen at 



a.nd Isocardia, it is curved like a hook or knee the base of the furrow of Mytilus edulis, orifices 



from behind in front. With Solen, it is very long, of the excretory ducts of this gland ; see Wieg- 



straight and nearly cyhndrical. mann\<> Arch. loc. cit. Taf. I. fig. 6. On the other 



i Cyclas and Pisidium. It is probable that those hand, neither J. Müller (De glandul. structura, p. 



species whose foot is furrowed upon its inferior bor- 39), with Tridacna ; nor R. fVagner (Lehrb. d. 



<ler (PectiinculuK), or bent in front (Nucula and vergleich. Anat. 1835, p. 271), with Area, and 



Trigonia), can also creep like the Gasteropoda. Pinna, has been able to find these glands. 



5 Pholas. 5 See A. Mailer, in fViegrnann''s Arch. loc. cit. 



1 The Malleacea, Aviculacea, Mytllacea, with Taf. I. fig. 5, c. (Tichosonia), and Poll, loc. cit. 

 Tecten, Lima, Area, Tridacna, &c. II. p. 132, Tab. XXIV. fig. 5-7 {Area). 



Quite singularly there exists with Anodonta, t> For the intimate structure of the byssus see 



Unio, and Cyclns, when hatched, a secretory the Memoir of A. Müller, loc. cit. With Area, its 



■organ of the byssus ; see below, § 197, note 13. form is very remarkaljle, consisting of a solid, lat- 



2 For the byssus-forming organ, see Dexkayes, erally-compressed trunk, carinated above and be- 

 Cyclop. of Anat. &c. I. p. 702 ; and especially A. low, and having filaments upon no portion. With 

 Müller, De Bysso Acephalorum, Dissert. Berolini. that of Pinna, on the contrary, its filaments remain 

 1836 ; or, his Memoir in fViegmann''s Arch. 1837, ununited even to the very root. 



I. p. 1, Taf. I. II. 7 Avicularia and Mytilus ; see Poli, loc. cit. 



3 The manner in which Mytilus and Tichogo- Tab. XXXI. (Mytilus edulis), and Tab. XXXIV, 

 nia act in spinning their byssus has been described fig. 2 (Pinna niuricsta). 



by Marion de Proce in the Ann. d. Sc. Nat. 

 XVUI. 1842, p. 59 ; and by A. Müller, loc. cit. 



