Messrs. Hancock and Eiiibleton on the Anatomy o/Eolis. 103 
does receive tranks of veins on each side and behind_, — tranks 
that result from the union of numerous venous branches of various 
size ; that it does not communicate directly with lacunje 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 arterial 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 phaenomenon 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 
papillae 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 5, 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 
covei'ing the whole of the under surface of the foot oi Doris and also of se- 
veral of the testaceous Gasteropods, there can be little doubt that they are 
present also on the foot of Eolis. 
