STETJCTIJEE AND rHYSIOLOGY OF TRE MOLLUSCA. 19 



inversion 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 j)n,tterns. In this class [tunkata) they act only as 

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

 expel the "^ater 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 [spicula), 

 which renders it a very efficient instru- 

 ment for the enlargement of their cells. 

 {Hancock). In the attached bivalves it 

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

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

 material of those threads with which the mussel and jr/???ia 

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

 hi/ssus ; the plug of the anomia and the pedicel of terebratula 

 are modifications of the hyssus. 



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

 cartilages which represent the bones of vertehrate 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 cytlierea (and other di-myaries), by 

 two ; the contraction of which brings the valves together. 

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



* Tlie muscular fibres of molluscs frequently present the transverse stripes -whiclj 

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

 observed in Salpa {Huxley); and in Waldhcimia australis by Hancock ; a strict search 

 was iiuide by that able anatomist for the purpose of di.-^covering such fibre amongst the 

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

 gasteropoda. 



t Fig. 13. Dreisseiia poJijmorpha (Pallas sp.), from Iha Surrey tinibc: -docks 

 f, foot ; b, byssus. 



