v GENERAL CHARACTERS 143 



of the coelom also surrounds the heart and forms a pericardial 

 cavity, and in some Craniata the genital ducts may be formed 

 from its lining membrane. 



There is always a muscular heart, consisting of at least three 

 chambers, a sinus venosus, an auricle and a ventricle, and formed 

 by a modification of the initial portion of the ventral or cardiac 

 aorta of the Cephalochordata. The disposition of the great blood- 

 vessels is based on a common plan in all Craniata, and the blood 

 which circulates in them is red in colour owing to the presence of 

 red, haemoglobin-containing corpuscles in addition to the colour- 

 less leucocytes which alone are present in the Cephalochordata. 

 Ductless blood-glands of various kinds (spleen, thyroid, thymus, 

 inter- and ad-renal bodies) are very generally present, and 

 modify in .different ways the character of the blood as it 

 circulates through them. Besides blood-vessels there is also 

 a somewhat similar system of lymphatic vessels distributed 

 throughout the organs and tissues of the body, which serves the 

 purpose of re-collecting the fluid portion of the blood that has 

 diffused from the blood-vessels for the nutrition of the tissues, 

 and conveying it back to the blood vascular system. These lymph- 

 atics contain lymph, a fluid comparable to dilute blood plasma, 

 in which leucocytes float. In addition to their continuity with 

 the blood-vessels at certain points, the lymphatic vessels may 

 also communicate with the coelom, and hence the Craniata must 

 be included among those somewhat rare exceptions to the general 

 rule that no connexion exists between the series of blood-con- 

 taining channels and the coelom. 



In the excretory system the renal tubuli in the adult Craniata 

 rarely retain their primitive embryonic communication with 

 the coelom, and in no instance have they separate and in- 

 dependent external apertures ; on the contrary, by the union of 

 their outer or distal extremities, common efferent ducts are formed, 

 which either open into a " cloaca," or directly on to the exterior of 

 the body near the anus. 



In all Craniates the dorsally-placed and tubular central nervous 

 system has its anterior portion enlarged and otherwise modified 

 to form a " brain," while the remaining portion, retaining a 

 simpler and more uniform structure, forms the spinal cord. In 

 the embryo the brain always consists of three successive sac-like 

 enlargements known as the fore-, mid-, and hind-brain, and from 



