86 



BALANOGLOSSIDA. 



proboscis. It extends from the hinder side of the notochordal 

 caecum in front to the junction of the notochord with the mouth 

 behind (Fig. 64, ri). At this point it divides into the two limbs 

 which diverge backwards into the collar region lying immediately 



within the buccal epithelium. The 

 limbs may be long, extending al- 

 most through the whole length of 

 the collar, or they may be short 

 reaching but a little way. The 

 proboscis skeleton is strengthened 

 laterally in the hinder part of the 

 neck of the proboscis by the chon- 

 droid tissue. This tissue is the 

 enlarged basement membrane be- 

 longing to the anterior wall of the 

 collar coelom and the posterior 

 part of the dorsal canals of the 

 FIG. 68. Proboscis skeleton of oios- proboscis coelom. It differs from 



sobalanus sarniensis (after Spengel). 



the rest of the skeletal tissue of the 



body in containing nests of cells which have migrated into it 

 from the adjacent coelomic epithelial walls (p. 84). The 

 chondroid tissue, which has a certain resemblance to 

 hyaline cartilage, is specially developed in Schizocardium and 

 Glandiceps. 



The heart is a blood vessel lying in the basement membrane 

 between the notochord and the pericardium (Figs. 65, 67). 

 Its connexions will be described later on when we are dealing 

 with the vascular system. The pericardium or proboscis sac is 

 a closed vesicle lying over the anterior end of the notochord and 

 extending only for a short distance backwards into the stalk 

 (Figs. 64, 65, 67). This posterior part of it is traversed by 

 transversely directed fibres ; its anterior part is filled up with a 

 peculiar loose tissue of unknown function while its main body 

 contains a clear fluid. It possesses an epithelial lining which in 

 some cases may proliferate so as almost entirely to fill up its 

 cavity. It has no connexion with the vascular system, and 

 the latest observers have asserted that it is quite closed off 

 from the coelom, but Bateson maintained that its cavity com- 

 municates with the coelom through the loose tissue filling up its 

 anterior part. This loose tissue sometimes occupies a large 



