III. 2 



MOVEMENT OF BALANOGLOSSUS 



5i 



('abdomen') is intact. The proboscis, collar, and trunk each contain 

 a coelomic cavity, and the coeloms of the proboscis and collar are 

 distensible by intake of water through a single proboscis pore and 

 paired collar pores. The skin is richly ciliated all over the body. The 

 outer epithelium is thus unlike the squamous, layered skin of higher 

 forms (Fig. 23). It contains numerous gland-cells, whose secretion 

 is very copious, so that the animals are always covered with slime. 



neur.s 



Fig. 23. Section of the epidermis of an enteropneust. 

 h.m. basement membrane; ep. epidermal cell; gl. I and 2, different types of gland cell; 

 neur. neuron; neur.s. neuro-sensory cell; n.g.p. process of epidermal cell acting as neuro- 

 glia in the nerve net. (After Bullock, v. der Horst and Grasse.) 



A characteristic feature is an unpleasant smell, resembling that of 

 iodoform, which possibly serves, like the mucus, as a protection. 



Below the skin is a nerve plexus receiving the inner processes of 

 receptor cells and containing ganglion cells (Fig. 23). Deep to this are 

 muscles running in various directions. It is said that the animal moves 

 by first pushing the proboscis and collar forward through the sand 

 and then drawing the body after it. Protrusion of the proboscis can- 

 not, however, be very vigorous. It may perhaps be produced by ciliary 

 action distending the coelom as is usually stated — more probably by 

 circular muscles, but these are weak. Numerous longitudinal muscles 

 are present, however, in the proboscis and trunk and are partly 

 attached to a plate of skeletal tissue in the collar. This tissue is 

 attached to the ventral side of a forwardly directed diverticulum of 

 the pharynx. The wall of this is thick, composed of vacuolated cells, 

 and bears a certain resemblance to a notochord (Fig. 24). A notochord 

 extending throughout the length of the body would clearly be dis- 

 advantageous for an animal whose main movements are lengthening 



