LYMPHATICS OF VERTEBRATA. 601 



passed over, althougli its relations to tlie lymphatic system are still 

 very uncertain; this is the thymus. This is an organ which is also 

 made up of glandular follicles, and which is divided into larger and 

 smaller lobes ; its smallest vesicles are filled with cells. In the 

 Selachii this organ is placed on the branchial sacs, and between them 

 and the dorsal muscles. In the Sturgeon, and some Teleostei, the 

 similar follicles that are found on the superior hinder boundary of 

 the branchial cavity are regarded as the same organ. In the 

 Amphibia the thymus is a small swelling, placed behind the angle of 

 the lower jaw. It has the same characters in the Keptilia. In the 

 Ophidii and Chelonii it is placed on the carotid, and above the heart. 

 In the Crocodilini, as in Birds (Fig. 312, tli), it extends from the 

 pericardium to the lower jaw. The lower portion is the larger in 

 the Mammalia, so that it rarely passes beyond the thoracic cavity. 

 In all cases it is best developed in early life, after which it undergoes 

 atrophy, and it is very rarely that it retains its earlier size in the 

 adult stage (Pinnipedia). 



In the higher divisions of the Vertebrata there is an organ which 

 lies in front of the kidneys and on either side of the body ; it is con- 

 sequently called the supra-renal gland, but we have no information 

 at all as to its function. In the Anamnia these structures are re- 

 placed by the investment of the sympathetic ganglia by means of a 

 cortical layer made up of cell- containing tubes ; these form yellowish 

 or whitish bodies, and are scattered over a larger portion of the 

 body, whereas in the Amuiota they form a mass on either side, 

 while nerve-elements can be made out in their medullary substance. 

 The relatively large size of this organ during fa3tal life is a note- 

 worthy point. The function of these organs cannot be regarded as 

 at all definitely known ; nor are we aided in our inquiry by classing 

 it as one of the " blood -vascular glands " — a term which is altogether 

 obscure, and consequently objectionable. 



Excretory Organs. 



§ 447. 



The arrangements which are found subserving the purpose of 

 excretory organs among the Invertebrata obtain also, in their most 

 essential relations, in the Vertebrata, and are therefore indications 

 of the affinity between the vertebrate phylum and lower forms, which 

 in other morphological details are very remote. Organs of this 

 kind have as yet been vainly sought for in Amphioxus ; but in all 

 the Craniota they are found to exist, and to be formed on the same 

 type. The type has been obliterated by gradual differentiation, 

 but it is revealed by the study of individual development. The 

 simplest stage is represented by a canal which runs in the dorsal 

 wall of the coelom, and opens to the exterior posteriorly, and in the 



