Mr. A. Hancock on the Anatomy of Oithona. 293 



slightly prolonged behind for the reception of the posterior por- 

 tion of the tongue, and the muscles are arranged much as in 

 Eolis. On the dorsal aspect they are extensively developed, 

 forming a dense mass, the fibres passing transversely and having 

 their extremities inserted into the dorsal margins of the plates. 

 These muscles undoubtedly assist in the motion of the jaws. 

 Those for moving the whole apparatus forward are composed of 

 flattened, isolated bands with their extremities attached to the 

 posterior margin of the plates and to the muscles forming the 

 walls of the channel of the mouth. 



The tongue is supported on a fleshy ridge that rises up from 

 the floor of the buccal cavity, and extends in the antero-posterior 

 direction from the oesophagus towards the anterior opening. 

 This organ (fig. 6) is long, linear, and strap-formed, and is 

 composed of forty transverse, semicircular plates (PI. X. fig. 7) 

 of an orange colour, each bearing a stoutish central spine and 

 six or seven smaller ones at the sides ; these latter having appa- 

 rently a minute denticle at the base of their outer margin. All 

 the spines are a little bent, and have their points directed back- 

 wards towards the (Esophageal opening. 



The coi-neous plates (PI. X. figs. 5, 6) are little short of the 

 size of the buccal mass, and much elongated, well arched and 

 ovate ; and, when united and entirely isolated, strongly resemble 

 the valves of a minute Mytilus. They are smooth, glossy, and of 

 a brownish amber colour, darkest towards the anterior extremity, 

 which gives support to the cutting blade [a). This is a wing- 

 like appendage of no great size, terminating below in a free 

 point, and having the cutting margin arched forward, plain, and 

 nearly at right angles to the general direction of the plate ; above 

 is a small process or fulcrum {b) — the point at which the two 

 plates are articulated; and immediately behind this point the 

 dorsal margin of the plates is reflected and expanded into an 

 arched lobe (c) for muscular attachment. 



The oesophagus (fig. 1 Z») is a short and rather slender tube, 

 which, passing from the upper aspect of the buccal mass towards 

 its posterior extremity, opens into the anterior margin of a distinct 

 pyriform stomach (c). This organ has the broad end foi-ward, is 

 placed above the reproductive apparatus, and lies quite in the 

 anterior p rtion of the visceral cavity. The internal sm-face of the 

 gastric organ does not appear to be lamellated. The intestine {d) 

 passes from the posterior extremity of the stomach, and inchning 

 slightly to the right side, passes backwards to the tubular anus 

 (PI. IX. fig. 2 a), which is placed a little to the right of the me- 

 dian line of the back, immediately behind the heart. The intes- 

 tinal tube is rather short, of equal diameter throughout, and in- 

 ternally plicated longitudinally. 



