CYCLOSTOMI. 



433 



tidae) develope a cuticular border pierced by fine pores. The deep cells 

 are prismatic and stalked. There are also (i) superficial goblet cells ; 

 (2) large oblong club cells, with a distinct membrane and lamellated 

 contents, which gradually reach the surface as they increase in size, and 

 there burst ; as well as (3) granular cells of unknown function, spherical or 

 ovoid in shape, with a membrane and nucleated granular contents, and fine 

 processes extending towards the corium. In Myxine. the Hag-fish, the 

 superficial cells of the epidermis are all goblet cells, two to three layers 

 deep, which give origin for the most part to the abundant mucus these 

 fishes throw off. There are besides large oblong cells with granular 

 contents, which reach the surface and burst ; and spherical ' spider ' cells 

 with clear contents, central stellate granular mass, and a distinct membrane. 

 These two kinds of cells appear to correspond respectively with the club 

 and granular cells of the Lamprey. Myxine possesses also on each side of 

 the body a series of pores leading into pit-like glands imbedded in the sub- 

 cutaneous tissue. They originate as solid ingrowths of epidermis. Their 

 cells are of two kinds * spider ' cells and thread cells the latter oblong or 

 ovoid, with remains of a nucleus at one pole, a central granular core, and 

 contents differentiated into a thread, wound transversely at the surface, 

 longitudinally in the deeper layers. As soon as the cell is mature, the 

 thread begins to unwind. The epidermis of Petromyzon Planeri contains 

 scattered rod cells, terminating in sensory hairs. P. fluviatilis possesses 

 end buds, each containing but a few cells, and on the head elevated upon 

 papillae. Nerve eminences, somewhat sunk in pits, are found in Petro- 

 myzon, arranged in an upper and lower lateral line, and the three typical 

 lines can be traced upon the head. Myxine has none of these structures. 

 There is a remarkable fatty subcutaneous tissue. The myomeres of the 

 body retain their primitive arrangement in Lampreys, but in Myxine the 

 ventro-lateral portion is broken up into an outer oblique layer, and into 

 ventral longitudinal but interrupted bundles. The myomeres overlap one 

 another in the Lampreys, and there are numerous longitudinal fibrous 

 septa stretching from one myocomma to the next succeeding. The com- 

 partments thus formed are filled by the muscle plates. 



The cranium is entirely cartilaginous. It has no roof in the Myxi- 

 noidei) and is only partially roofed in the adult Lamprey. It always 

 retains a large basicranial fontanelle. There appear to be well-developed 

 palatine, pterygoid, quadrate, and hyomandibular regions, a hyoid arch, 

 and in Myxinoidei traces of two branchial arches. The Lamprey is said 

 to have distal rudiments of Meckel's arch. A large basi-hyo-branchial bar 

 supports the tongue. It is carried by the hyoid arch, with which it is 

 continuous in Myxinoidei^ and by which it is ' clamped ' in Petromyzon. In 

 the latter there are five upper labial cartilages, one median and two paired, 

 and an annular cartilage supports the margin of the mouth. It represents 



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