﻿236 CONTRIBUTIONS TO THE NATURAL HISTORY 



the contracted body is restored to its expanded form. Upon the inner surface there is 

 another system, which contracts the sphere, acting in antagonism with the former. These 

 two systems consist of bundles extending vertically from the upper portion of the vault 

 downwards. Within the inner vertical system, there is another one consisting of con- 

 centric transverse bundles, lining the cavity of the body, the direction of which tends to 

 reduce the capacity of the space inclosed between the walls of the animal and the lower 

 partition. A fourth system of circular concentric bundles is spread through the whole 

 partition below. This system, in its strongest contractions, may shut almost entirely the 

 main cavity of the body, and, like the pupil of the eye, it opens and shuts constantly. In 

 its less powerful contractions, it assists the inner transverse and vertical muscles in redu- 

 cing the capacity of the inner cavity, and when deeply contracted it helps more fully than 

 any other part of the contractile system in forming the body into a sphere. We have 

 thus here four distinct muscular systems, an external superficial system, an inner system 

 parallel to the former, a concentric system of the main cavity, and a concentric system 

 of the partition below. 



It will be worth while to examine more minutely the arrangement of this muscular 

 apparatus, as otherwise it might be difficult to form correct ideas of the movements de- 

 scribed, and perhaps be supposed that the very existence of these muscles was not al- 

 together so satisfactorily ascertained, if their arrangement could not be traced in detail. 



Let me say, in the first place, that the outer coat of the animal consists of a layer of 

 flat polygonal nucleated cells, forming, as it were, the external point of attachment for 

 the external muscular system. Indeed, to this coating the superficial muscular system 

 seems to be suspended, penetrating more or less into the gelatinous mass. The muscles, 

 or rather the bundles of elongated cells, do not converge upon the summit. There is a 

 circle, or rather a polygon of fibres (Plate V. Fig. 3), occupying about one third of the 

 summit, from which vertical bundles run down to the lower margin. There are four 

 main bundles of the kind, alternating with the eyes when seen from below. (Plate V. 

 Fig. 4, a, and 3, a.) These bundles are the strongest of the external system; and when 

 powerfully contracted, and especially when assisted by the inner corresponding fibres, 

 they give the whole body a four-lobed appearance. They act in accordance with the in- 

 ner system only in its most powerful contractions. In a state of repose, when the body 

 is relaxed and the muscles slightly contracted, they however constitute a sort of antag- 

 onism with the bundles of the inner surface of the gelatinous mass. Besides these bun- 

 dles, we have eight more following the same direction (Fig. 3 and 4, b, b), and occur- 

 ring, also, in the intervals between two nutritive tubes, accompanying, therefore, at some 

 distance, the main bundles already described, and bearing an evident relation to them, 



