Watt. — On Boltenia pachydermatina. 



337 



in which the stalk at the proximal end was greatly thickened, 

 the delicate anterior prolongation of the body, instead of run- 

 ning straight into the stalk, formed for a little distance a very 

 close spiral. 



The greatest amount of variation exists with regard to the 

 length of the stalk, which is always greater than that of the 

 body. To illustrate this I will give a few examples : — 



Length of Body. Length of Stalk. 



When the test has been removed the body is seen lying 

 within its delicate anterior prolongation, proceeding for a little 

 way into the stalk. This prolongation narrows away gradually 

 distad till at last it ends in a fine point. The apertures (oral 

 and atrial) are at the end of tubular prolongations of the body, 

 which are surrounded by powerful sphincter muscles. In the 

 fresh state the body is almost transparent, with the exception 

 of the bright-orange digestive gland, the dull-green gonads, 

 and the posterior part of the intestine, which, from the sand, 

 fine mud, &c., it contains, is always of a dark colour. 



In fresh specimens the body is kept in close connection 

 with the test by the ectoderm which secretes the latter ; but 

 in specimens preserved in spirit the mantle (muscular tunic) 

 shrinks away from the ectoderm and test, the only connection 

 being at the apertures and at the place where the two blood- 

 vessels to the test leave the body. 



B. Muscular Tunic (Mantle). 



The second layer is the muscular and connective-tissue part 

 of the body- wall. It is formed of connective tissue uniting and 

 enclosing the bundles of muscular fibres and blood-vessels, 

 and in the region of the dorsal surface also the nerves. 



The muscular fibres are arranged in two layers — an ex- 

 ternal circular round each of the oral and atrial siphons, and 

 an internal transverse. 



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