Messrs. Hancock and Embleton on the Anatomy o/Eolis. 103 



does receive trunks of veins on each side and behind, — trunks 

 that result from the union of numerous venous branches of various 

 size ; that it does not communicate directly with lacunas among 

 the viscera is also certain ; and that if we admit the existence of 

 lacunae, they do not supersede the venous system, but occupy 

 the position of the capillary system of the higher animals. With 

 regard to the artei'ial system, we can follow M. de Quatrefages 

 with confidence only so far as the bifurcation of the aorta, and 

 have not been able to discover the symmetrical division and ar- 

 rangement of its branches as described in his memoir and figured 

 in his plate, but we have succeeded in tracing many branches of 

 arteries to a degree of fineness of which that gentleman seems to 

 entertain no idea. 



M. de Nordmann describes a ventricle and funnel-shaped pro- 

 cesses, but besides these mentions an auricle ; in other respects 

 he seems to have fallen into the same errors as M. de Quatre- 

 fages : these errors seem due to the exclusive use of the compressor. 



Organs of Respiration. 



The function of respiration we believe to be performed by the 

 whole surface of the skin, including the papillae; the skin of the 

 back and of the sides between the papillae, and the entire sur- 

 faces of these latter organs, present the phenomenon of ciliary 

 vibration*. The papillae we regard as one modification among 

 many of increasing the surface for a respiratory purpose, and 

 thus are to be regarded as a specialized breathing apparatus, to 

 which the rest of the skin is subsidiary. 



The skin, PI. IV. fig. 5, consists of a layer of muscular fibres 

 covered by a tegumentary envelope or cutis that is provided with 

 an epithelium. 



The skin varies much in thickness in different parts, being 

 thinnest over the back and on the papillae, very thick where the 

 papilla? exist ; and it here contains near the external surface the 

 ramifications of the digestive system, becomes much thinner sud- 

 denly where the papillae cease along the sides, and attains on the 

 foot its greatest thickness and strength. Its epithelium consists 

 of very small granular nucleated particles, which during life are 

 provided with vibratile cilia. 



The outer or dermal layer of the skin, fig. 5 b, appears to 

 secrete the abundant tenacious matter that exudes from the ani- 

 mal, and to be the seat of an exquisite sensibility ; this layer is 

 thin, but continuous with the next or muscular layer a, which 



* Having recently, and since writing the above, discovered vibratile cilia 

 covering the whole of the under surface of the foot of Doris and also of se- 

 veral of the testaceous Gasteropods, there can be little doubt that the}' are 

 present also on the foot of Eolis. 



