72 ORIGIN OF CHORDATES in. 8- 



which develops within the mother without passing through a tadpole 

 stage, nourished by a diffusion placenta, whose cells also migrate into 

 the developing embryo. This becomes the asexual oozoid and pro- 

 duces a long chain of blastozooids, which it tows about until these 

 break away by sections (Fig. 40). 



The pelagic colonial Pyrosoma of warm seas consists of a number of 

 individuals associated to form an elongated barrel-shaped colony. 

 The mouths open outwards and the atria inwards into a single 

 cavity with a terminal outlet from which a continuous jet emerges. 



The mode of budding from the epicardium 

 and other features suggest an affinity with 

 Doliolum and Salpa, but Pyrosoma also 

 resembles the ascidians in that its zooids 

 are all sexual and capable of budding. The 

 yolky eggs develop within the parent, 

 without forming a larva. The outstanding 

 characteristic of the creatures is the 

 powerful light that they shine. This is 

 Fig. 41. Photogenic cell of produced in photogenic organs on each 



Pyrosoma. (After Kukenthal.) r . r mi 1 



side or the pharynx. 1 he photogenic cells 

 contain curved inclusions about 2^ in diameter (Fig. 41). These 

 are considered by some to be symbiotic luminescent bacteria, but 

 this is doubtful. The light is so powerful that when large masses of 

 Pyrosoma occur together the whole sea is illuminated sufficiently to 

 allow of reading a book. A remarkable feature of the phenomenon 

 is that the light is not produced continuously but only when the animal 

 is stimulated, as by the waves of a rough sea. If one individual is 

 stimulated others throughout the colony may show their lights, but 

 the mechanism of this effect is not known and the groups of cells that 

 form the luminescent organs receive no nerves. Other types of animal 

 with luminescent bacteria emit light continuously. The sudden flashes 

 of light probably serve as a dymantic reaction (p. 302), giving protec- 

 tion against enemies by producing a flight-reaction in the same way 

 as do sudden manifestations of colour or black spots by other animals. 

 It has been observed in the laboratory that colonies of Pyrosoma that 

 are dying and do not light up may be eaten by fishes, whereas any 

 that light up when seized may then be dropped. 



9. Class Larvacea 



The (Appendicularia) Larvacea (Figs. 42 and 43) are minute neo- 

 tenous tunicates that live in the plankton. Instead of the test, each 



