LATERAL LINE. MUSCLES. 97 



not communicate in the adult with the abdominal part of the 

 body cavity, and the pronephros does not persist. In the 

 Myxinidae the nasal sac does communicate with the alimentary 

 canal by an aperture which perforates the roof of the mouth, the 

 eyes are much reduced and without the muscles and the cor- 

 responding cranial nerves, the pericardium communicates with 

 the general body cavity by a wide opening on the right side, 

 and the pronephros is persistent in the adult. Moreover, the 

 Myxinidae possess a contractile dilatation on the portal vein 

 (portal heart) which is not present in the lampreys. 



The skin is slimy, and has the usual vertebrate structure. It 

 possesses unicellular glands which secrete the mucus. In the 

 Myxinidae there is in addition on each side of the body and 

 embedded in the subcutaneous tissue, a row of segmentally 

 arranged slime-glands, which open on 

 the surface and pour out a mucus 

 containing an immense number of 

 threads. These threads arise in special 

 cells of the gland and unwind them- 

 selves when the mucus is discharged. 

 They were discovered by Retzius and , 

 described and figured by Miiller. z^"' 



Nothing of the nature of lateral line 

 sense-organs has been observed in fig. 49.— Thread-ceii ofiifyzme 



Ti/f • -J Ui-'i-i 1 11 glutinosa with unwinding 



Myxinidae, but m the lampreys small tiiread (after Muiier). 



sensory eminences, partially sunk in pits, 



are found on the head and in two double rows on the body.* 



The great lateral muscles are divided up by septa, which have 

 a zig-zag course, into myomeres of the usual piscine type. The 

 myomeres extend on to the head to just behind the eyes. In 

 the Myxinidae there is in addition a ventral sheet of obliquely 

 directed muscle- fibres which is unsegmented. There is a 

 complicated system of muscular bands connected with the 

 mouth, tongue, and pharynx. 



In Petromyzon the portion of the lateral muscles dorsal to the gill-sacs 

 is continued to just behind the eye and contains a greater number of seg- 

 ments than the corresponding ventral portion. The ventral part reaches 

 to just in front of the first gill opening. In Ammocoetes there is one 

 myomere anterior to the first gill aperture ; this in the adult divides up 

 into nine or ten myomeres (Schneider). 



* Langerhans, op. cit. 

 z— II H 



