22 STUDIES IN BIOLOGY. 



Arteries of the labial palps, or tentacular arteries. First, to define 

 the terms applied to the sides of the palps, in i I p, and o I p, Figs. 1 

 and 2, the inner or lower sides of the palps are shown : this side I 

 shall call the ventral side ; the upper or outer side will then be called 

 the dorsal side. Most of the arteries to the palps spring from 

 the anterior side of a branch of the anterior aorta, this branch 

 lying transversely in the floor of the supra-eesophageal cavity (tr a, 

 Figs. 24 and 27), behind the anterior ends of the anterior 

 retractors. Each palp has two arteries, one on the dorsal side, 

 the other on the ventral side, and these tentacular arteries run 

 along near the anterior edge of the palps. The tentacular arteries, 

 rising then from a branch of the aorta, run forward in the floor 

 of the supra-sesophageal cavity, some of them passing under the 

 commissure between the cerebral ganglia (Fig. 27) to the outer 

 edge of the upper lip. The dorsal tentacular arteries of the outer 

 palps can be seen thence to pass directly round the outer edge of 

 these palps, being superficial in all its length (Fig. 27). The ventral 

 tentacular arteries of the outer palps, and the dorsal tentacular 

 arteries of the inner palps, pass together down by the sides of the 

 mouth, being lost to sight until they appear near the anterior edges 

 of their respective palps (Fig. 28 : in this plate the dorsal tentacular 

 artery of the inner palps cannot be seen, but it exactly resembles 

 that figured on the ventral side of the outer palps) . The ventral side 

 of the inner palps is supplied by arteries from the anterior ventral 

 artery as previously described {a v a, Figs. 24 and 28), but, as this 

 anterior ventral artery is part of the same branch of the aorta as that 

 which gives rise to all the other tentacular arteries (tr a, Fig. 24), 

 the whole blood-supply of the palps is derived from the same branch 

 of the aorta. 



To explain the exact course of the arteries in the palps, it is 

 necessary to refer to the structure of the palps themselves. The outer 

 palps have a smooth dorsal surface, while the inner palps have a 

 smooth ventral surface. The sides of the palps facing each other — 

 namely, the ventral side of the outer palps and the dorsal side of the 

 inner palps — have a similar and peculiar structure. The anterior 

 margin has an edging of short grooves perpendicular to the margin 

 itself and parallel to each other. Along the posterior ends of these 

 grooves there runs a longitudinal groove, enclosed by a longitudinal 

 ridge or flap, which also overlaps the posterior ends of the transverse 

 grooves. The arteries of these grooved sides — that is, the ventral 

 artery of the outer palp and the dorsal artery of the inner palp — run 

 in this longitudinal groove at a little distance from the edge, and these 



