56 LOWER VERTEBRATES. 



Sub-Class I. — Tethyodea, 



Most of the members of this group are attached in the adult condition, though 

 they pass through a stage in their development when they are free-swimming and 

 tadpole-shaped. One of the most important features common to all is, that in the 

 course of their metamorphosis the body is so folded that the mouth and atrial aper- 

 tures are brought close to each other, and are not placed at opposite ends of the 

 body. 



Many of the group are solitary ; that is, during the whole of their life each indi- 

 vidual remains distinct and separate from all others. Next come the so-called social 

 ascidians, in which the original parent sends off buds which develop into new individ- 

 uals, each with its own organs, but still connected to the parent. In the last tj'pe, the 

 compound ascidians, the condition is variously complicated. The original individual 

 gives rise to buds which develop into new zooids ; but these, instead of being distinct, 

 as in the social forms, become united into one mass with more or fewer of the organs in 

 common. We shall recur to these aarain. 



Order I. — COPELAT^. 



In this, the lowest of the orders of tunicates, features characteristic of the larva of 

 other groups are retained in the adult. The species are all minute, free-swimming 

 animals, which in the course of their development never pass beyond the tadpole stage. 

 In the tail, the notochord and neural cord persist through life, the latter with gan- 

 glionic enlargements at regular inter\als, which is accompanied by a segmentation of 

 the mesoblast. Only a single family, x^ppendiculaeidje, is recognized, and the 

 genera are few. Oikopleura (or Appendicular ia) is a genus common on the surface 

 of the seas far from land, both in the tropics and in more temperate regions. Two 

 species have been noticed by Mr. A. Agassiz on the southern shore of New England, 

 but they have not yet been described. 



Among the other peculiarities of these animals we may mention that a cloaca is 

 not formed, the intestine communicating directly with the water, and only two gill 

 slits are present. Some of the species form a transparent gelatinous protective en- 

 velope, known as the test or house. This is secreted very rapidly, and when complete 

 has two anterior apertures, while from the middle arises a spacious chamber in whicli 

 the tail has ample room to vibrate. According to Fol this house is occupied for only 

 a few hours, and then the animal deserts it and forms another. 



Placed in this family is a very rudimentary form, called by Fol JRowalevsIcia, after 

 the eminent Russian naturalist. In this genus there is no heart and no endostyle, while 

 the intestine is wanting. The only known species comes from the Mediterranean. 



Order II. — MONASCTDI.E. 



The term ascidians is frequently employed as synonymous with tunicates, but it is 

 better restricted to embrace the social and solitary forms, the Ascidise Simfilices of many 

 authors, and the ' sea squirts ' of the language of the shore. The family Ascidiid^ 

 includes the solitary forms, the general structure and development of which was suffi- 

 ciently elucidated in our general account. This family embraces the largest species of 



