CLASS CH^TOGNATHA. 187 



ectoderm lies a well-defined basement-membrane, and below 

 this are tbe muscles of the body-wall, which are, as a rule, 

 longitudinal in their direction, and are interrupted, as in the 

 Nematodes, along four longitudinal lines, one dorsal, one ven- 

 tral, and two lateral. In one species of Spadella there is on 

 the inner side of the longitudinal muscles a thin layer of trans- 

 verse muscles, but usually only longitudinal fibres are present, 

 except in the head, when there are a number of special muscle- 

 bundles for the movement of the jaw-bristles. 



Within the musculature of the body-wall is the spacious 

 coelom, lined throughout by a delicate layer of cells constitut- 

 ing the peritoneum, and divided into three chambers by 

 transverse partitions, one of which lies just behind the head, 

 while the other is towards the hind end of the body. The 

 peritoneal epithelium lines the surfaces of these dissepiments, 

 and in the trunk and tail regions is reflected in the mid- 

 dorsal and ventral lines towards the centre of the body, form- 

 ing a mesentery, surrounding the intestine and dividing the 

 coelom into lateral compartments. 



The mouth lies on the ventral surface of the head and 

 opens into an oesophagus surrounded by a single layer of 

 muscle-fibres having a dorso-ventral direction and passing 

 above and below into the general musculature of the head. 

 After being narrowed in passing through the anterior dissepi- 

 ment the digestive tube again expands (Fig. 92, i), and is sup- 

 ported throughout the trunk region by the mesentery. In 

 this region it is a simple straight tube, unprovided with mus- 

 cle-fibres, and terminates in an anal opening situated ven- 

 trally at the junction of the trunk and tail regions, not being 

 continued into the latter. 



Neither an excretory nor a blood vascular system is pres- 

 ent. The nervous system lies for the most part imbedded in 

 the ectoderm, and consists of two principal ganglionic masses, 

 of which one, the cerebral or supraoesophageal ganglion (ce), 

 lying in the head region, is situated in the ectoderm of the 

 dorsal surface of the body and has a somewhat hexagonal 

 outline, giving off five pairs of nerves, one pair passing back- 

 wards as commissures to unite with the ventral or sub- 

 cesophageal ganglion, lying also in the ectoderm a little in 



