OP ARTS AND SCIENCES. 101 



the eighth, ninth, and tenth segments present the most favorable 

 opportunity for studying the contraction of the blood-vessels. When 

 one of these vessels is distended, its walls are seen to contain large, 

 prominent nuclei, evidently belonging to muscular elements (PI. I. 

 fig. 7). When contracted the muscular elements are seen to be much 

 shortened and thickened. The walls of the vessels also show longi- 

 tudinal and transverse striae. The vessels contract rapidly and are 

 distended much more slowly. Their contractions take place at differ- 

 ent rates in different individuals, and under different conditions. The 

 observed limits are 11 and 24 contractions per minute. Although the 

 ventral vessel does not contract, its walls contain the muscular ele- 

 ments mentioned above, but in less number. 



No trace of blood corpuscles is seen in the living animals, but in 

 sections the blood-vessels are frequently found filled with a granular 

 mass apparently containing many corpuscles. This appearance may 

 be due, however, to the action of reagents on the blood. 



The Respiratory Organs. 



The function of respiration is performed, in great part at least, by 

 the pavilion. This and the distribution of blood-vessels to it have 

 been already described. It is thickly covered with cilia, which pro- 

 duce an inward current of water. It contains numerous branched 

 muscular elements, so arranged as to cause it to contract and close. 

 The digitiform appendages are hollow, their cavities being continuous 

 with the general body cavity. Their walls (PL III. figs. 29, 30) are 

 composed cf a single layer of dermal cells covered by the cuticula, 

 which is beset with hairs (not shown in the figure). The cavity of 

 each appendage contains many branched muscular cells, the processes 

 of which are attached to the walls of the appendage. 



Respiration is doubtless also carried on through the richly-ciliated 

 walls of the intestine, which are covered by a network of blood-vessels 

 and bathed by a steady stream of water. 



The Nervous System. 



The nervous system consists, as in related forms, of (1.) a supra- 

 oesophageal ganglion and (2.) two commissures joining this to (3.) the 

 ventral cord. The first two parts make up the circum-cesophageal ring. 



The supra-cesophageal ganglion, or hrain, (PI. II. figs. 11, 19, 20; 

 PI. III. figs. 21, 25-27,) is situated immediately above, and in front 

 of, the point where the mouth cavity joins the pharynx, and is divided 

 by a median superior and anterior fissure into lateral halves, each of 



