320 DR W. CARMICHAEL M'lKTOSH ON THE STRUCTURE OF THE 



homogeneous though complex layer, which, doubtless, has its physiological use 

 in the varied movements of the organ. The next layer (d, same figures) consists 

 of a strong coat of longitudinal fibres, fully twice as thick as the external longitu- 

 dinal layer, and which may be termed the inner longitudinal muscular coat. In 

 essential structure it resembles the exterior, differing only in bulk. In sections 

 prepared by hardening in alcohol, these fibres, in common with others in this 

 organ, present a much coarser appearance in transverse section than after harden- 

 ing in chromic acid. It may be mentioned also, that there is a considerable 

 histological difference between these muscular fibres and those in the higher 

 animals, such as absence of nuclei and greater homogeneousness. The fifth 

 layer from without inwards is a strong band of circular fibres (c, same figures), 

 the circular muscular coat, which forms a counterpoise to the preceding. . Lying 

 on the inner side of these fibres is a basement-layer of pale translucent texture, best 

 observed in the longitudinal sections (Plate IV. fig. 4). where it is marked h. In 

 transverse sections this coat is apt to be confounded with the inner layer of 

 circular fibres, but the distinction between the two is sufficiently apparent in 

 longitudinal sections. It has, on the whole, a cheesy or cartilaginous aspect. 

 Upon this layer rest the peculiar glandular papillae, which arise from a distinct 

 margin on its inner edge, as indicated at b in the last-mentioned figure, where 

 some of the basal streaks of the papillae are represented. A glance at the other 

 figures will show the relations and proportions of these organs. In the ordinary 

 transverse sections of the proboscis they form en masse a somewhat foliated or 

 frilled arrangement, often more strictly symmetrical than the view here given (Plate 

 V. fig. 4). In some contracted specimens they block up the entire cavity, or else a 

 transparent mucous film which has exuded from them does so. The form of the 

 glands in the fresh specimen under pressure is seen in 0. alba in Plate V. fig. 7, and 

 in Tetrastemma in Plate V. figs. 6 and 11. The largest glands are situated some 

 distance in front of the stylets, for towards this region they become smaller, and 

 finally the fundus is clothed only by minute papillae. In typical examples of 

 Tetrastemma variegatum the glandular papillae are leaf-shaped, and somewhat 

 crenated at the free border, where there is a regularly streaked appearance from the 

 arrangement of the globules. Under pressure they are granular in the interior, 

 and furnished with numerous globular or wedge-shaped mucous masses, that 

 refract the light like oil. Sometimes in O. alba they present a coarsely fringed 

 appearance, with large granules in their interior ; and when the tube has been 

 turned inside out, they have a villous aspect, the tough mucosity adverted to 

 above projecting in strings from the papillae under the slightest pressure. I have 

 generally observed also, towards the first portion of the protruded organ, fine 

 motionless processes like cilia projecting from the apices of the glands, and they 

 are probably homologous with the minute spikes which occur on the glands of 

 the posterior region after rupture from pressure. 



