VERTEBRATA: FISHES. 479 



branchial filaments before mentioned. The water- passes 

 through the slits in the branchial sac, and thus gains access 

 to the abdominal cavity, from which it escapes by means of 

 an aperture with contractile margins situated a little in front 

 of the anus, and called the " abdominal pore " (/). There is 

 no distinct heart, and the circulation is entirely effected by 

 means of rhythmically contractile dilatations which are de- 

 veloped upon several of the great blood-vessels. In other 

 words, the heart retains its primitively tubular condition, and 

 special contractile dilatations are developed upon other vessels 

 (those carrying the blood to the pharynx). The blood itself 

 is colourless. No kidneys have as yet been certainly identi- 

 fied, and there is no lymphatic system. There is no skeleton 

 properly so called. In place of the vertebral column, and 

 constituting the whole endoskeleton, is the semi-gelatinous 

 cellular notochord (n), enclosed in a fibrous sheath, and giving 

 off fibrous arches above and below. The notochord is, further, 

 peculiar in this, that it is prolonged quite to the anterior end 

 of the body, whereas in all other Vertebrates it stops short 

 at the pituitary fossa. There is no cranium, and the spinal 

 cord does not expand anteriorly to form a distinct cerebral 

 mass. The brain, however, may be said to be represented, 

 since the anterior portion of the nervous axis gives off nerves 

 to a pair of rudimentary eyes, and another branch to a ciliated 

 pit, believed to represent an olfactory organ. The generative 

 organs (ovaria and testes) are not furnished with any efferent 

 ducts (oviduct or vas deferens). The generative products, 

 therefore, are shed into the abdominal cavity, and gain the 

 external medium by the " abdominal pore." 



ORDER II. MARSIPOBRANCHII ( = Cyclostomi, Owen ; and 

 Cydostomata, Muller). This order includes the Lampreys 

 (Petromyzonidcz) and the Hag-fishes (Myxinidcz), and is defined 

 by the following characters : The body is cylindrical, worm- 

 like, and destitute of limbs. The skull is cartilaginous, without 

 cranial bones, and having no lower jaw (mandible). The noto- 

 chord is persistent, and there are either no vertebral centra, or 

 but the most rudimentary traces of them. The heart consists of 

 one auricle and one ventricle, but the branchial artery is not 

 furnished with a bulbus arteriosus. The gills are sac-like, and 

 are not ciliated. 



The type of piscine organisation displayed in the Marsipo- 

 branchii is of a very low grade, as indicated chiefly by the 

 persistent notochord without vertebral centra, the absence of 

 any traces of limbs, the absence of a mandible, and the struc- 

 ture of the gills. 



