STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 25 



instance, when it draws in its eye-stalks, by a process like the 

 inversion of a giove-finger ; the branching gills of some of the 

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

 nently contractile.* 



The inner tunic of the ascidlans (fig. 8, t.) presents a beau- 

 tiful 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 re- 

 traction (fig. 30 h) ; its structure and mobility has been com- 

 pared to that of the human tongue. In the burrowing shell-fish 

 (such as soleu), it is very large and powerful, and in the boring 

 species, its surface is studded with silicious particles (spicula), 

 which render it a very efficient instrument for the enlargement 

 of their cells. (Ha7icock.) In the attached bivalves it is not 

 developed, or exists only in a rudimen- 

 tary state, and is subsidiary to a gland 

 which secretes the material of those threads 

 with which the mussel and pkna attach 

 themselves. (Fig. 13.) These threads ^ 'j ., ^ 

 are termed the bi/ssus ; the plug of the ^ iu ^ 



anomia, and the pedicel of terebratula 

 are modifications of the byssus. 



In the cuttle-fishes alone, we find muscles attached to in- 

 ternal 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, the equivalents of the 

 pectoral fins of fishes. 



* The muscular fibres of shell-fish do not exhibit the transverse stripes 

 which characterize voluntary muscles in the higher animals. 



t Fig. 13. Breissena poli/morpha (Pallas sp.) from the Surrey timber- 

 docks. /, foot, b, byssus. 



C 



rig. 13. Dretssena.^ 



