152 MANUAL OF ANIMAL BIOLOGY 



material which is so very abundant in plant tissues. Solitary 

 types (Ciona, Molgula) and colonial types (Amaroucium) are 

 abundant in marine waters as shown in figure 9. Reproduction 

 may occur either sexually or asexually (budding). Alternation of 

 generations is also known in some species. 



C. Cephalochordata. Only one genus comprising a few ma- 

 rine species are classified in this subphylum, but included among 

 these is the important species Branchiostoma lanceolatus, more com- 

 monly known as Amphioxus, or the Lancelet. The importance 

 of Amphioxus from the zoological aspect lies in its possession 

 throughout life of structural features which link it closely with the 

 true Vertebrates as well as with the Chordates. It is a small fish- 

 like animal a few inches in length and is able to dart about quite 

 rapidly when disturbed. The adult, however, usually lies verti- 

 cally in a sand burrow with only the anterior end of the body 

 projecting. As in the case of the Tunicates, noted above, food is 

 secured from the water current which continually bathes the gills. 

 Amphioxus lacks a definite head, jaws, and limbs. There is a 

 dorsal fish-like fin which runs the length of the body in the median 

 line, enlarges near the posterior end to form a wide caudal fin and 

 continues forward ventrally for a short distance. There is no heart, 

 but a well-developed system of blood vessels is present, some of 

 which are contractile. The main flow of blood, which is towards 

 the anterior end ventrally and towards the posterior dorsally, 

 is similar to that of the Vertebrates. (W. f. 67.) 



D. Vertebrata. We may now turn our attention to a con- 

 sideration of certain general features associated with the Verte- 

 brata, by far the most important subphylum of the Chordates, 

 including as it does all the familiar animal types such, for example, 

 as the Fishes, Frogs, Snakes, Birds, Rabbits, and Man. Certain 

 important diagnostic features of this subphylum — in addition 

 to the three fundamental Chordate characteristics — may now 

 be briefly noted (W. fs. 94, 95) : 



(1) As indicated by the term vertebrate and noted above, 

 all these forms possess a backbone, or vertebral column. This 

 important supporting structure is composed, except in the most 

 primitive vertebrates, of a considerable number of bony segments, 

 or vertebrae, which develop in close relation to the unsegmented, 

 cartilaginous notochord of the embryo and, usually, entirely sup- 

 plant it in the adult. 



