STRTJCTFEE AND PHYSIOLOGY OF THE MOLLUSCA. 19 



in version of a glove-finger ; the branching gills of some of 

 the sea-slugs, and the tentacles of the cuttle-fishes are also 

 eminently contractile.* 



The inner tunic of the ascidians (Fig. 8, t) presents a beautiful 

 example of muscular tissue, the crossing fibres having much 

 the appearance of basket-work ; in the transparent salpians, 

 these fibres are grouped in flat bands, and arranged in charac- 

 teristic patterns. In this class [tunicata] they act only as 

 sphincters (or circular muscles), and by their sudden contraction 

 expel the water from the branchial cavity. The muscular foot 

 of the bivalves is extremely flexible, having layers of circular 

 fibres for its protrusion (Fig. 18, /), and longitudinal bands for 

 its retraction (Fig. 30 *) ; its structure and mobility has been 

 compared to that of the human tongue. 

 In the burrowing shell-fish (such as 

 solen) , it is very large and powerful, and 

 in the boring species, its surface is 

 studded with siliceous particles {spieula), 

 which renders it a very efiicient instru- 

 ment for the enlargement of their cells. 

 {Hancock). In the attached bivalves it 

 is not developed, or exists only in a rudi- ^ig- ^^- DreissenaA 

 mentary state, and is subsidiary to a gland which secretes the 

 m.aterial of those threads with which the mussel and pinna 

 attach themselves (Fig. 13). These threads are termed the 

 hyssus ; the plug of the anomia and the pedicel of terehratula 

 are modifications of the hyssus. 



In the cuttle-fishes alone we find muscles attached to internal 

 cartilages which represent the bones of vertebrate animals ; the 

 muscles of the arms are inserted in a cranial cartilage, and those 

 of the fins in the lateral cartilages. 



Muscles of a third kind are attached to the shell. The valves 

 of the oyster (and other mono-myaries) are connected by a 

 single muscle; those of the cytherea (and ocner di-myaries), by 

 two ; the contraction of which brings the valves together. 

 They are hence named adductors; and the part of the shell 



* The muscular fibres of molluscs frequently present the transverse stripes which 

 characterise voluntary muscles in the higher animals. Striped muscular fibre has been 

 • observed in Sal pa {Huxley); and in Waldheimia austrnlis by Hancock; a strict search 

 was made by that able anatomist for the purpose of discovering such fibre amongst the 

 liingeless brachiopods, but without success. Striped fibres have been seen in the 

 gasteropods. 



t Fig. IS. Breissena polymorpha (Pallas sp.), from the SuiTey timber-docks. 

 f, foot ; b, byssus. 



