22O 



CLASSIFICATION OF VERTEBRATES. 



dorsals and caudal. The skin is without scales, but is rich in 

 mucus-secreting cells, and in the myxinoids contains also nu- 

 merous pockets of so-called 

 thread cells, these pockets 

 extending into the underly- 

 ing muscles. These thread 

 cells have their protoplasm 

 FIG. 223. Thread cells of Bdeilostoma, converted into a long thread, 



one intact, the other ' exploded,' after 

 Ayers. 



and when these are dis- 

 charged, the threads become 



unwound ; and these and the mucus are so abundant that one 

 of these animals will convert a bucket of water into a thick jelly. 



The mouth is at the bot- 

 tom of a more or less circular 

 suctorial funnel, the inside of 

 which, like the tip of the 

 mobile tongue, is armed with 

 horny, cuticular teeth, which 

 aid the suckers in anchoring 

 these animals to the fish on 

 which they feed, and also serve 

 to rasp the flesh. The alimen- 

 tary canal is straight ; a cir- 

 cular fold, the velum, occurs 

 inside the mouth ; no cloaca is 

 present. The brain has well- 

 developed cerebral lobes which 

 may be solid or hollow, but 

 the cerebellum is very small. 

 The nasal organ is median, and 

 is placed at the posterior side 

 of the hypophysial duct, the 

 opening to which thus serves 

 as the nostril, opening either 

 at the tip of the snout (myxi- 



FIG. 224. 



Petromyzon. 



noids) or upon the top of the 

 head (petromyzontes). The 

 deeper end of the hypophysis expands into a large sac, which 



Brain and nasal organ of 

 In front is (darker) the 

 nasal canal, behind which are the plaits of 

 the nasal membrane. On either side of 

 the twixt brain are bits of the cartilage 

 of the chondrocranium, and farther back 

 the otic capsules. 



