170 GEPHYREA [CH. 



to the intestine, and when the critical period of growth has arrived, 

 the hood or prostomium is cast off : the ventral pouch becomes 

 turned inside out and pulls out the intestine into a loop and this 

 everted pouch constitutes the greater part of the body of the 

 Phoronis. 



The largest of the Sipunculoidea is the species Sipunculus 

 nudus which is found burrowing in the sand and mud in the 

 Mediterranean. It is however used as a type for study by students 

 of zoology in this country, and as species of the same genus occur 

 on the American coast we may give a brief description of its 

 anatomy and its habits. The animal is shaped somewhat like a 

 sausage, and may attain a length of 10 11 inches and a diameter of 

 nearly an inch. The front part of the body which can be retracted 

 and which we may name the introvert is covered with minute 

 papillae. The rest of the body is marked by a series of longitudinal 

 grooves crossed by transverse ones so that the skin is divided into 

 a series of squares. There is a well-developed cuticle somewhat 

 loosely attached to the ectoderm beneath, and in the ectoderm are 

 a series of glands, each of which consists of a number of oval cells 

 arranged in the form of a shallow cup. The mouth is situated 

 at the end of the introvert and is surrounded by what can only 

 be described as a frilled membrane (a, fig. 69). This is represented 

 in other genera by a horse-shoe of ciliated hollow tentacles lying 

 dorsal to the mouth. This membrane is richly supplied with blood- 

 vessels and seems to serve as gatherer of the minute particles of 

 organic matter on which the animal feeds. The blood-vessels 

 communicate with a main reservoir, the dorsal blood-vessel (c, fig. 69) 

 which lies above the oesophagus. When blood passes from this 

 into the membrane the latter becomes swollen out. 



The animal burrows its way into muddy sand and feeds on it as 

 well. If it is removed from the sand it turns and twists the 

 introvert about, alternately retracting it and pushing it out until 

 it finds sand of the proper consistency, when the introvert is driven 

 in. It then becomes fixed in its new position by the appearance of 

 a swelling on it immediately behind the mouth. It is then retracted 

 and this action since the introvert is fixed necessarily draws the 

 body after it. When the body is fixed the introvert is again 

 extruded and so movement goes on. 



When a Sipunculus is opened by a cut along the dorsal surface we 

 find that we have cut into a spacious coelom undivided by any 

 septum whatever. The most conspicuous organs which strike the 



