ANATOMY 187 



which is most prominent at the sides (Fig. 102, g) ; within 

 the hood the head bears from two to four rows of short 

 spines, and outside these a right and left row of sickle-shaped 

 hooks, the free ends of which in a state of rest converge round 

 the mouth, but when disturbed these hooks can be widely 

 divaricated. 



The cavity of the body, or coelom, is divided into three dis 

 tinct chambers by the presence of two thin transverse walls or 

 septa, one situated between the head and the trunk, the other 

 between the trunk and the tail (Figs. 104, 105). In the head, 

 this cavity is much reduced by the presence of special muscles 

 which move the spines, hooks, etc. ; and in the small species, such 

 as Spadella cephaloptera, the other two cavities are almost entirely 

 occupied by the digestive and reproductive organs * ; but in the 

 large species, e.g. Sagitta hexaptera, a considerable space is left 

 between the internal organs and the skin, and this is occupied by 

 a coelomic fluid. If the skin of one of these larger species be 

 punctured the fluid escapes and the animal shrivels up. A 

 longitudinal partition or mesentery, with numerous pores in it, 

 runs through these spaces, dividing the body-cavity into a 

 right and left half; in the region of the trunk this mesentery 

 supports the alimentary canal 



In addition to certain muscles in the head, which mdve the 

 hooks, etc., there is a muscular lining to the body-wall. This is 

 divided into two dorsal and two ventral bands, much in the same 

 way as in Nematodes. The muscle fibres are striated. 



The mouth, situated either terminally Spadella marioni 2 

 or below the head, leads into a pharynx ; this passes into an intes- 

 tine lined by a single layer of ciliated cells with a few glandular 

 ones intermingled. The intestine runs straight through the 

 body without loop or coil, and opens by an anus situated at the 

 junction of the trunk and the tail. In most cases the anus is 

 ventral or on the lower surface, but Gourret asserts that in Spadella 

 marioni it is on the upper surface. 



There are no special respiratory, excretory, or circulatory 

 organs, unless a glandular structure described by GouiTet in the 

 head of Spadella marioni be a real kidney. 



The nervous system consists of a supra-oesophageal ganglion 



1 0. Hertwig, Jen. Zcitschr. Bd. xiv. 1880, p. 196. 

 2 P. Gourret, Ann. Mus. Marseille, torn. ii. Mem. 2, 1884, p. 103. 



