500 LECTURE XX. 



prehension are greater ; and Cuvier suspects that it may even enjoy 

 a species of locomotion from the superior length of its peduncle. The 

 organisation of its mouth and stomach indicates the molecular 

 character of its food ; but its convoluted intestine shows a capacity of 

 extracting a quantity of nutriment proportioned to its superior 

 activity and to the greater extent of its soft parts. The more ex- 

 tended pallial respiratory apparatus is in exact harmony with the 

 above conditions of structure and habits. 



With regard to the Orbicula, and more especially the deep sea 

 species of Terebratula, both the respiration and nutrition of such 

 animals which exist beneath a pressure of from sixty to ninety 

 fathoms of sea-water, are subjects suggestive of interesting reflec- 

 tions, and lead one to contemplate with less surprise the great 

 strength and complexity of some of the minutest parts of the frame of 

 these diminutive creatures. In the unbroken stillness which may 

 pervade those abysses, their existence must depend upon their power 

 of exciting a perpetual current around thera, in order to dissipate the 

 w^ater already laden with their effete particles, and to bring within 

 the reach of their prehensile organs the animalcules adapted for their 

 sustenance. The actions of the Terebratula and Orbicula, from their 

 attachment to foreign bodies, are confined to the movements of their 

 brachial and branchial fringe cilia, and to a slight divarication and 

 sliding motion of their protecting valves ; the simplicity of their 

 digestive apparatus, the still greater simplicity of their branchiae, 

 and the diminished proportion of their soft to their hard parts, are in 

 harmony with such limited powers. The soft parts, in both genera, 

 are, however, remarkable for the strong and unyielding manner in 

 which they are connected together. The muscular system is much 

 more complex than in ordinary bivalves, and is remarkable for the 

 compactness of its fibre and the density of the glistening tendons. 

 Here is obviously an apparatus of sufficient power to effect the 

 requisite motions of the valves at the depth of which they may be 

 destined to live. The Terebratididce and Spiriferidce have an internal 

 skeleton superadded to the bivalve defensive covering, by means of 

 which additional support is afforded to the shell, a stronger defence 

 to the viscera, and a firmer basis of attachment to the spiral arms. 



Class TUNICATA. 



Body unsymmetrical, with an uncalcified tegument in form of a 

 tunic, having two openings, and lined by the mantle ; oral tentacles 

 rudimentary or absent ; no foot. 



Generation by gemmation alternating with impregnated ova. 



