452 The Animal Kingdom 



are the layers of muscles. The first of these is the thin layer of circular 

 muscles; below this are the longitudinal muscles which are arranged 

 in four tracts, two dorsolateral and two ventrolateral. Oblique mus- 

 cle fibers which originate in the median ventral line pass into the 

 lateral portion of the body wall. The parietal peritoneum forms the 

 inner membrane of the body wall. 



The coelom is divided into segments by the septa, thin sheets of 

 connective tissue. Dorsal and ventral mesenteries extending from the 

 body wall to the intestine divide the segments into right and left 

 halves. 



The mouth which is located ventral to the prostomium opens into 

 a buccal cavity. Posterior to the buccal cavity is the muscular, pro- 

 trusible pharynx. Two large jaivs are present in the posterior por- 

 tion of the pharynx. These jaws are well supplied with heavy mus- 

 cles. This portion of the digestive tract is known as the joregut and 

 can be everted like a proboscis. When protruded, numerous small 

 denticles and the widely opened jaws are visible. Behind the pharynx 

 is the esophagus which leads into the long stomach-intestine. The 

 esophagus is located in segments 7 to 12 and is supplied at its anterior 

 end with two large caeca which may have a digestive function. The 

 long stomach-intestine is slightly constricted in each segment. 



There are two principal blood vessels in Neanthes; one, the mid- 

 dorsal, acts as a heart to pump the blood forward ; the other, the 

 midventral, carries the blood posteriorly. 



In each segment of the body except the first and last are paired 

 metanephridia. The ncphridiopores are located on the ventral surface 

 of each parapodium. The nervous system is essentially like that de- 

 scribed for the phylum. The sense organs are highly developed in 

 Neanthes. The tentacles, palpi, and cirri are probably all tactile. The 

 four eyes are quite complex in structure. 



The members of this genus are dioecious, but there are no spe- 

 cialized organs for the production of the gametes. These simply ap- 

 pear as seasonal outgrowths from the peritoneal wall. The eggs or 

 sperm mature within the coelomic cavity and are released either by 

 rupture of the body wall or through the ncphridiopores. Fertilization oc- 

 curs outside the body and the embryos develop into trochophore larvae. 



The Trochophore Larva. — The trochophore larvae (Fig. 145,C) 

 is a developmental stage found in the marine representatives of some 

 phyla, including the molluscs and annelids. 



