ON THE DIPLOCHORDA. 349 



proximal area of the collar. In the trunk it is thinner, though 

 slightly thickened in the ventral sucker. It is only present 

 in connection with the ''somatic^' part of the mesoderm in 

 contiguity with the ectoderm, except in the case of the gonads 

 and the lateral masses. 



At the inner posterior end of the gonads there is a thick wall 

 of this tissue surrounding their tapering extremities. The 

 structure of this skeletal tissue is the same throughout, a 

 hyaline supporting but elastic mass, staining easily. There 

 can be little doubt that it is mesoblastic in origin, considering 

 that parts of it, such as around the gonads and in the lateral 

 skeletal bodies, are far removed from the ectoderm or endoderm, 

 and that the tissue in question is in such intimate connection 

 with the muscular system. In this case it could be homologised 

 Avith the chondroid tissue of Phoronis, and even with the 

 mesoblastic cartilaginous skeleton of the Vertebrata. 



In the regions in which there is no chondroid tissue, such 

 as the trunk mesenteries and mesodermic (splanchnic) layer 

 over the endoderm, the mesoderm appears in the form of a fine 

 protoplasmic layer (PI. 25, figs. 18, 19, sh.) , with nuclei here and 

 there. In the parts directly enclosing blood-sinuses there is often 

 a thickening of this layer, due to an accumulation of branched 

 cells, which are probably contractile (see Vascular system). 

 It is possible that a thin layer of mesoderm may line the 

 coelomic cavities internally to the chondroid tissue, but the 

 protoplasmic filaments have the appearance of actually arising 

 from the substance of the chondroid tissue. 



With the exception of the muscle-fibres of the ventral 

 sucker, therefore, the skeleto-muscular system of Cepha- 

 lodiscus appears to remain in the undifferentiated condition 

 of protoplasmic strands or branched cells and a hyaline 

 skeletal matrix, as in the early stages of other forms, such as 

 Phoronis. 



Mcintosh (3) has suggested that the elasticity of the chon- 

 droid tissue (" basement membrane ") may compensate for the 

 absence of circular muscles in the ventral sucker and other 

 parts, and, in addition, it evidently imparts a rigidity to the 



