TUNICATES. 59 



Order IV. — LUCIiE. 



In this group are placed a few species of compound ascidians which form thimble- 

 shaped, hollow colonies, the individuals radiating from the axis and being imbedded in 

 a common cartilaginous matrix. The incurrent openings are irregularly disposed on 

 the outer surface, while those for the outgoing stream empty into the hollow of the 

 thimble. 



Only a single family, the Pyeosomid^, is recognized, and of this the principal genus 

 is JPyrosoma, which derives its name from its highly developed phosphorescent 

 powers. The members of the genus are found in the warmer seas of the world, where 

 they swim slowly through the water. Their mode of locomotion, if such it may be 

 called, is rather peculiar. Each individual is constantly drawing the water in through 

 the branchial openings on the outside, and pouring it out through the cloaca into the 

 central cavity, and thence back into the sea. This constant streaming forces the 

 whole colony constantly forward in one direction ; but there is no adaptation for 

 changing the course. Pyrosoma reproduces in two ways, — by eggs and by budding. 

 The eggs hatch and leave the parent, and form the bases of new colonies, which are 

 built up by budding from the zooid produced from the egg. 



Pyrosoma, as we have said, is highly phosphorescent, and the larger colonies 

 (P. giganteum reaches a length of two ^r three feet) emit a large amount of light ; in 

 fact, the story is told that one naturalist read a descriistion he had written of one of 

 these animals by the light emitted from its body. Each of the individuals has a 

 number of cells near the mouth, the function of which is to produce the light. 



Sub-Class II. — Thaliacea. 



The members of this division present marked difference from those of the last. All 

 are free-swimming and perfectly transparent, and have the incurrent and excurrent 

 orifices at opposite ends of the body. The branchial cavity is much altered, and the 

 gills take a cylindrical or lamelliform shape, and project into the cavity, dividing it, 

 in Doliohcm, into two chambers. Expiration is assisted by a number of muscles like 

 hoops, which, by their contraction, force the water out of the cavity, and at the same 

 time the reaction forces the body forward. The alimentary tract forms a small body 

 (called the nucleus) at one side of the branchial cavity. 



Order I. — CYCLOMYARIA. 



The name used to designate this order, which contains only the family Doliolidje, 

 has reference to the character of the muscles which surround the body. Each one 

 forms a complete hoop. The body is barrel-shaped, with an opening at either end, 

 and the alimentary tract has not attained the concentrated condition found in the 

 next order. The reproduction is accompanied by an alternation of generations, 

 numerous cases of which are described in the lower invertebrates, but which here and 

 in the next order obtains its only representation in the group of vertebrates. The 

 whole of the history is not known : but essentially it is as follows: "We have first a 

 sexual form which produces eggs. From each egg a tadpole-like larva hatches out 

 and develops into an asexual individual. This in turn gives rise to buds which 



