OF THE INVEUTEBUATE ANIMALS. 33 



which are extremely interesting in this point of view ; the former 

 as being intermediate between the Mollusca and the Crustaceaj 

 the latter as forming the transition of the former to the Zoophytes. 



3. Cirripeds. 



I have examined Lepas {Itevis). The stalk and extremities 

 (cirri)j when treated with potash in the manner so frequently men- 

 tioned, become colourless and transparent; so do the branched, 

 jointed and simple hair-cells : they prove to be tubes of chitine, 

 serving for the protection and support of the numerous muscles 

 governing their segments, which latter are arranged in a sheath- 

 like form, and in which the former play. The inner surface of this 

 chitine-tube is covered with a layer of pigment cells, resembling 

 those of the choroid coat, and such as also covers the concave 

 surfaces of the articulated calcareous shells lying next the body, 

 which, corresponding to the analogous coverings of the bivalves, 

 nevertheless appear to be joined together by chitine ligaments, 

 i. e. Crustacean ligaments. The analogy of these articulated cal- 

 careous shells with those of the Conchifera is evident from the 

 following analyses : — 



r766 dried at 248° F. after being heated to redness, and de- 

 .ducting the amount of carbon left on solution, gave I"7ll5 in- 

 combustible residue, containing 0'012 phosphate of lime. 

 Hence the shells contained per cent. — 



Albuminates .... 3*09 



Incombustible residue . 96*81 

 and 100 parts of the latter contained — 



Carbonate of lime . . . 99*30 



Phosphate of lime . . . 0*70 

 The above-mentioned albuminate remained m the form of 

 structureless white films after treatment with dilute acids, just 

 as in Unio ; but the calcareous shells of Lepas are not furnished 

 with the horny investment on the outer surface of the Anodontee 

 (hardened duplicature of the mantle), but at this period are un- 

 attached to the last-formed calcareous layer (or rather the earliest 

 formed calcareous lamellae). 



Thus, even in a purely chemical point of view, the Cirripeds 

 retain their position in the animal kingdom. 



4. Ascidia. 



These animal forms, which in regard to the history of their 

 development have been too little investigated, present us with 



VOL. V. PART XVII. D 



