BDELLOSTOM-l IHlMBEVI, LAC. I 37 



as there are gills. After entering these sacks the water is 

 forced out through the sides of the body through other short 

 tubes which connect the gill-sacks with the surface of the skin. 

 Here they form the circular gill-holes to be seen on the side of 

 the head region of the animal. Two features in the arrange- 

 ment of the gills deserve attention. First, the large tongue- 

 muscle seen in Figs. 5, 9 extends backwards among the gills, in 

 this individual separating the anterior five pairs, effectually pre- 

 venting any contact between the gill-sacks of the opposite 

 sides and also bringing about a compression of the sacks in this 



Fig. r_ \ side view of the cephalo-branchial region of Bdellostoma. Some 



of the mternal organs are drawn into the figure, the body walls being thought 

 transparent. The arrow shows the direction of the respiratory current of water 

 toward the nose, r i, 2, 3, and 4, the tentacles. Th, the teeth, which here are 

 erected and thrown outwards by the unfolding of the tongue. X, olfactory organ. 

 E, the ear. /, the eye. J/, the club-shaped tongue-muscle. E, the external 

 branchial pores. Tr, the thread gland pores. S. C, the spinal cord. A', the 

 kidneys. /A the intestine. Z, liver. H, heart, Below and behind each gill 

 opening is seen the opening of a thread gland, u, above the figure, the single, 

 median, pharyngeal tooth. 



region whenever the tongue-muscle is in action ; second, the 

 apparent relation of the point of bifurcation of the branchial 

 aorta to the posterior end of this tongue-muscle. The number 

 of gills varies to an astonishing degree, and the significance of 

 this variation has not heretofore been the subject of any serious 

 inquiry so far as I am aware. There are two series of facts 

 concerning this flexibility of the branchial system which 

 deserve separate consideration. First, the number of gills of 

 individuals from different localities varies from 6 on either 

 side to 14 on either side, with the observed intermediate 

 stages as follows: 



