TRITONIA. 



ridge on their outer margin. The membrane of T. plebeia is broader than in T. alba, but 

 not by any means so broad as in T. Hombergii. In it there are thirty-two rows of simple 

 spines with the points much and suddenly hooked : each row contains forty-two of these, and 

 a central and two lateral plates like those of T. alba. 



The muscular support of the dentigerous membrane is composed of two sets or layers of 

 fibres, — an external and an internal layer ; these move over each other freely, there being- 

 no union between their surfaces. The inner layer (fig. U) forms the walls of the cavity 

 within which the posterior portion of the dentigerous membrane is placed; and being 

 attached in front to a dense, semi-cartilaginous body, upon which the spiny membrane is 

 expanded, and behind to the posterior part of the inner surface of the jaws, this layer will, 

 when brought into action, draw together the sides of the tongue, bringing the spines to close 

 upon the food. The outer layer is made up of three strata of fibres, by far the thickest of 

 which 0', i) underlies the other two, and is formed of numerous transverse laminae, arranged 

 in regular parallel order. These laminae or fibres act also upon the anterior semi-cartilaginous 

 support, and have their opposite extremities attached to the inner surface of the jaws, 

 immediately in front of the attachment of the inner layer. This stratum is for the purpose 

 of rotating the tongue backwards, and upwards in the direction of the gullet, and will, 

 with the aid of the inner layer, withdraw the whole organ. Immediately overlying this 

 stratum is another (j) the fibres of which are most developed in front or below, and cross 

 those of the under stratum. These fibres pass likewise from the semi-cartilagenous support, 

 and go to the root of the tongue, and are apparently for the purpose of rotating the organ 

 forward. The third or superficial stratum is very thin; its fibres are transverse, and have a 

 tendency to compress the tongue, and perhaps in this way to elongate it. 



The lingual muscles in Doris and Eolis are arranged much in the same manner, modified 

 only to suit the altered condition of the parts. In Doris tuberculata the semi-cartilagenous 

 support of the dentigerous membrane is developed to a much greater extent ; and the inner 

 and outer layers of muscles, which are not so distinctly marked, have both their extremities 

 attached to it, and the muscular fibres move over it as cords over a pulley. The semi- 

 cartilagenous support does not appear to exist in Eolis, and the muscles are no longer divided 

 into inner and outer layers. But a powerful mass of fibres is provided for drawing the tongue 

 back, which is attached by one end to the posterior extremity of the dentigerous membrane, 

 and by the other to the jaws. The stratum of fibres, for rotating the tongue forward, appears 

 to be represented in that genus by the muscles, which, forming the sides of the lingual mass 

 in front, are attached by one end to the anterior portion of the spinous membrane, and, inclining 

 backwards have their other extremity inserted into the jaw behind. By the interlacement of 

 the fibres of these latter muscles, a dense mass is formed in the centre of the tongue, which 

 gives support to the spiny ridge, and firmness to the whole organ, and acts as a sort of 

 fulcrum about which the retractor muscles work. 



The oesophagus (fig. 2b) in T. Hombergii is rather wide, of moderate length, and passes 

 from the upper surface of the buccal mass further forward than usual. On each side of it 

 along, tubular, folliculated salivary gland {c, c) opens into the mouth. The stomach (d), of no 

 great size, is irregular in form, and is partially concealed by the liver, the left lobe of which 

 lies over it. The intestine (figs. 1 and 2e) is a short widish tube of equal diameter throughout ; 

 it passes from the upper aspect of the gastric organ, and advancing a little forwards, bends 



