82 THE TREMATODA 



spines scattered over the entire surface of the body (Fig. XIX. 

 2), or carried, in fihopalophorus, on two great tentacle -like 

 processes of the anterior end of the body (Fig. XI. 7), while in 

 Echinostoma the oral sucker is armed with spines (Fig. XI. 3). 

 These spines may be compared with those of Enantia, amongst 

 Turbellaria, and, like the cuticle, appear to be chitinous; they 

 aid in attachment, and perhaps in obtaining blood. The cuticle 

 presents two or three layers, differing in optical characters ; there 

 are no pore canals. Below the cuticle is a slight, granular 

 " subcuticula " ; below this come the circular muscle fibres of the 

 somatic musculature. Deeper still are gland cells, amongst the 

 longitudinal muscles, and parenchyma cells. 



There are three chief views as to the nature of this " invest- 

 ing membrane " : (a) It is a metamorphosed, cellular epidermis 

 (Zeller, Ziegler, Biehringer, Braun, etc.) ; nuclei have been stated 

 to occur in it in various members of the group (Grosterostomum, 

 Amphistomum, Monostomum), and in cercariae the .nucleated epi- 

 dermis is stated to become a cuticle, (b) It represents a basement 

 membrane, the true epidermis and cuticle having been cast off 

 (Schneider, Kerbert); this view is founded on the fact that, 

 during the development of Malacocotylea, an external layer of 

 ciliated cells is shed, and in later stages a cuticle-like membrane 

 remains, (c) The investing membrane is a cuticle in the same 

 sense as that of a Chaetopod. But here again differences of 

 opinion as to its origin exist : (a) Some believe that the " sub- 

 cuticula" serves as its matrix, and represents an epidermis which 

 has lost its cellular character ; (/3) others regard this " inter- 

 muscular subcuticula" as the most external layer of the par- 

 enchymal tissue (Braun, etc.), and that the gland cells alone are 

 the representatives of the epidermis. 



Recent studies by Blochmann (8, 9) and Kowalevski (23) 

 upon the structure of Cestodes and Trematodes go to show that 

 the investing membrane consists of two parts the greater part 

 of it represents a true cuticle, while the lowest layer is a basement 

 membrane (Fig. XIX. 2). The epidermis which is more clearly 

 seen in some Cestodes (Ligida) than in Trematodes is represented 

 by deep-lying cells, some of which are glandular, with narrow 

 necks traversing the basement membrane ; the cells of the 

 epidermis have, however, become separated from one another by 

 the upward growth of the parenchymal tissue and muscles, just as 

 in Hir-udo (Lankester, 25) and in some Oligochaeta (Benham, 6) 

 the blood-vessels with connective tissue invade the epidermis 

 and, penetrating between the cells, break up -the layer ; in these 

 Annelids the cells remain attached to the cuticle by a broad external 

 end ; but in Trematodes the invasion of tissue has gone so far as 

 to leave only a very narrow part of each cell in connection 



