122 



COMPARATIVE ANATOMY 



CHAP. 



The cutis in Atlanta is still comparatively thin. The network of muscles lying 

 immediately beneath it is not more strongly de\'eloped than in other Gastropods. 

 A special system of crossing muscle fibres independent of the other dermal muscula- 

 ture lies on each side under the cutis of the 

 ' ^"^ fin. This is the case in all Scteropoda. 



The integument greatly increases in 

 thickness in the typical Helcropoda 

 (Cariiutria, Pterotrachca), and the sub- 

 cutaneous muscular tube also grows 

 thicker. Over the body the latter con- 

 sists of two superimposed layers of fibres 

 crossing one another diagonally. In the 

 outer layer, the fibres run from above 

 anteriorly downwards posteriorly, and in 

 the inner from below anteriorly to above 

 posteriorly. On the head and snout, the 

 visceral dome and the tail-like metapo- 

 dium, the diagonal fibres of both layers 

 run longitudinallj'. In addition, an ex- 

 ternal circular musculature is found in 

 Carinaria nearly all over the body, in 

 Pterotrachca only in the snout. 



Turning now to Carinaria, which 

 still possesses a delicate, easily detachable 

 shell covering the visceral dome, but un- 

 able to protect any otlier part of the body, 

 we find the columellar muscle still per- 

 sisting in the form of two bands descending from tlie visceral dome into the fin and 

 radiating out to its edge. 



In Pterotrachca, where the shell is wanting and the visceral dome rudimentary, 

 the columellar muscle is also reduced. It has now no connection witli the visceral 

 dome, and commences half-way up the body wall as three short bands running down 

 into the fin and radiating out to its edge. 



The columellar muscle, which originally served for drawing the foot back into 

 the shell, now serves chiefly to bring about the lateral movements of the vertical 

 rowing fin into which the foot has been transformed. 



erti/ 



Fig. 100. — Patella, from above, after removal 

 of the shell (after Lankester). c. The separate 

 bundles of the shell muscle, the section of which 

 is horseshoe-shaped ; I, pericardium ; Ix, fibrous 

 septum behind the same ; n, digestive gland ; nil, 

 intestine ; k, larger right nepliridiuiii ; t, smaller 

 left ditto ; c, mantle border, widening anteriorly 

 into the mantle fold ; ccr, em, edge of mantle. 



2. Opisthobranchia. 



The columellar muscle is well developed in forms possessing a shell 



into 



which the body can be partly or wholly withdrawn. Where, however, the shell 

 is rudimentary or wanting, as is the case with most Opisthobranchia, the 

 columellar muscle atrojjhies or perhaps forms part of the pedal musculature. The 

 subcutaneous dermo - muscular tube, on the other hand, develops in jiroportion 

 to the activity of the animal. It consists of longitudinal, circular, and diagonal 

 muscle fibres, which occasionally form a regular network. The pedal musculature 

 is merely a thickened portion of this dermo-muscular tube in which longitudinal 

 fibres predominate. The development of the musculature varies nmcli in detail. 

 "Where movable or contractile dorsal appendages, gills, oral lobes, oral discs, para- 

 podia, etc., are developed, their musculature is detached from the dermo-muscular 

 layer, and the latter, in combination Avith the occasionally tough skin, forms a 

 passive organ of support for the former. 



A columellar muscle is further found in the Ptcropoda thecosomata. It is ventral 



