UROCHORDA OR TUNIC ATA. 



527 



above the surface. These habits clearly provide the reason why the delicate 

 gill-slit region of the body should be protected from injury and enclosed 

 within a special chamber, formed by the union of a pair of fin-like ridge.s. 



Other representatives of the group, closely resembling the European 

 lancelet, are found in shallow seas all over the world. A peculiar type 

 {Asymmetron hicayamim), which possesses a slender backward prolongation 

 of its tail, is found in the Bahamas, where it may be taken swimming freely 

 at the surface of the sea at night, or in the day-time buried in the coral sand. 



SUB-CLASS II.— UEOCHORDA OR TUNICATA. 



This group of exclusively marine animals is closely related to the Cephalo- 

 chorda, although its various representatives are very unlike Amphioxus in 

 their final shape and appearance. They agree with Amphioxus, however, in 

 having similar organs for breathing and feeding, and in having a similar 

 tubular nervous system and supporting rod along the back of the body. Un- 

 like Amphioxus, however, the primitive backbone or notochord of Tunicata 

 is confined to the hinder part of the body, which is in the form of a well- 

 marked tail like that of a tadpole. The body also possesses the power of 

 producing a gelatinous envelope or tunic called the test, which clothes the 

 animal either loosely as in Appendicularians, or closely as in all others. The 

 group consists of two principal sections, which may be called the Tailed and 

 Tailless Tunicata. 



The members of the first section (Appendicularians) are free-swimming, 

 and retain their tails with the nerve-cord and notochord during their whole 

 career. They are very small and inconspicuous creatures, 

 the body being about the size of a small pin's head, and the 

 tail less than a quarter of an inch in length, and the whole 

 animal is of glass-like transparency. The tunic is thin, and 

 forms a delicate "house" in which the Appendiculariai) 

 swims about, and from which it can escape when attacked by 

 its enemies. These creatures can often be caught at the sur- 

 face of the sea round our coasts by the use of a fine muslin 

 net. 



The members of the second sec- 

 tion, the Tailless Tunicata, are _ 

 tailed only in their early stages. 

 After a brief free-swimming career 

 they lose their tails, like the tad- 

 poles of the common fiog, and grow 

 up into soft back-boneless animals 

 of a quite different appearance. Some of these 

 tailless forms retain, or regain, their free-swimming 

 habits, and usually acquire a barrel-like shape and 

 a number of hoop-like muscle bands, by the contractions of which they drive 

 themselves through the water (Salps), or they may form definite colonies and 

 float idly about at the mercy of the waves (Pyrosoma). The majority of the 

 Tailless Tunicata, however, become permanently attached to rocks, stones, 

 and sea-weeds on the sea-bottom, and are especially abundant between tide- 



Fig. 2.— An Ap- 



PENDICULARIAX 



{Oikopleura). In 

 its house. 



Fuj. 3.— DOLIOLUM Tritonis. 



