Gatty Marine Laboratory, St. Andrews. 51 



all round. This dorsal pit, still retaining the dorsal groove 

 with modified cells on each side, then disappears, but it comes 

 near the central nervous system, and perhaps performs a 

 sensory function. Immediately thereafter the central 

 nervous system occupies the region above the gullet — sepa- 

 rated therefrom by strands of connective tissue with several 

 apertures. The gullet has an internal lining of columnar 

 nucleated cells which stain deeply, surrounded by a circular 

 muscular coat and an external investment of reticulated 

 tissue and nucleated cells. It is slung by several bands to 

 the coelomic wall around it, and instead of its cavity, now 

 diminished, having its long axis transversely placed, it is 

 vertical. Below it is the commissure between the oesopha- 

 geal ganglia, above it is a large transverse space in which 

 the dorsal vessel by-and-by appears, and the common duct of 

 the thoracic glands occurs below the hypoderm above it, and 

 blood-vessels lie internally. The investing cells and tissue 

 increase in bulk, and the cut ends of numerous vessels are 

 intermingled, whilst median furrows give a cruciform aspect 

 to the central cavity in section, and longitudinal muscular 

 fibres are more distinct within the circular coat. Below it 

 is the ventral blood-vessel, in the median line. The nuclei 

 of the coelomic cells are distinct and correspond with those 

 investing the alimentary canal. In this region (thoracic) 

 the dorsal and ventral blood-vessels are of large size, and the 

 rete around the alimentary canal well developed as a ring of 

 longitudinal vessels in section (PI. VI. fig. 35). The alimen- 

 tary canal now increases in size, and, in the preparations, 

 shows a tendency to split into layers, the entire lumen being 

 filled up by the various coats. Instead of the firm circular 

 coat with a few longitudinal fibres between it and the 

 columnar epithelial layer characteristic of the smaller 

 oesophagus, the area in section enlarges, the circular coat be- 

 comes thinner, the longitudinal investment within it thickens, 

 as also does the cellular mucous layer, and there is a ten- 

 dency to separation of these coats in the sections — indeed, 

 it is clear that a change is taking place in the structure of 

 the walls of the gut, probably representing a differentiated 

 stomach, the central part in the sections representing the 

 invaginated gullet and the larger separated external region 

 the stomachal wall. The latter consists internally of a 

 closely arranged, almost fibroid, cylindrical epithelium of 

 uniform thickness, then of the longitudinal fibres, followed 

 by the thin circular coat. The foregoing coats are invested 

 by the cellular and a vascular coat, which presents two 

 variations, tor the smaller region in front shows the cut ends 



4* 



