$$ 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- 
veloped, 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 stiffness 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 
have, 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 Tellina, Donaz, and Cyclas, it is very long 
and more or less ridged, and often quite small at 
its base. 
With Cardium, Nucula, Trigonia, Mactra, 
aand Isocardia, it is curved like a hook or knee 
from behind in front. With Soden, it is very long, 
Straight and nearly cylindrical. 
4 Cyclas and Pisidium. It is probable that those 
species whose foot is furrowed upon its inferior bor- 
der (Pectunculus), or bent in front (Nucula and 
Trigonia), can also creep like the Gasteropoda. 
5 Pholas. 
1 The Malleacea, Aviculacea, Mytilacea, with 
Pecten, Lima, Arca, Tridacna, &c. 
Quite singularly there exists with Anodonta, 
Unio, and Cyclas, when hatched, a secretory 
organ of the byssus ; see below, § 197, note 13, 
2 For the byssus-forming organ, see Deshayes, 
Cyclop. of Anat, &c. I. p. 702; and especially 4. 
Miller, De Bysso Acephalorum, Dissert. Berolini. 
1836 ; or, his Memoir in Wiegmann’s Arch. 1837, 
I. p. 1, Taf. I. I. 
3 The manner in which Mytilus and Tichogo- 
nia act in spinning their byssus has been described 
by Marion de Procé in the Ann. d. Sc. Nat. 
XVIII. 1842, p. 59; and by A. Mudler, luc. cic. 
’ 
4 I do not yet clearly understand the true nature 
of the walls which secrete the byssus. A. Miller 
has designated them as Glandula byssipara, com- 
posed of round cells. He aftirms to have seen at 
the base of the furrow of Mytilus edulis, orifices 
of the excretory ducts of this gland; see Wieg- 
mann’s Arch. loc. cit. Taf. I. fig. 6. On the other 
hand, neither J. Mtiller (De glandul. structura, p. 
39), with Tridacna ; nor R. Wagner (Lebrb. d. 
vergleich. Anat. 1835, p. 271), with Arca, and 
Pinna, has been able to find these glands. 
5 See A. Muller, in Wiegmann’s Arch. loc. cit. 
Taf. I. fig. 5, c. (Tichogonia), and Polz, loc. cit. 
II. p. 132, Tab. XXIV. fig. 5-7 (Arca). 
6 For the intimate structure of the byssus see 
the Memoir of A. Miller, loc. cit. With Arca, its 
form is very remarkable, consisting of a solid, lat- 
erally-compressed trunk, carinated above and be- 
low, and having filaments upon no portion. With 
that of Pinna, on the contrary, its filaments remain 
ununited even to the very root. 
7 Avicularia and Mytilus ; see Poli, loc. cit. 
Tab. XXXI. (Mytilus edulis), and Tab. XXXIV. 
fig. 2 (Pinna muricata). 
