552 



THE BIOLOGY OF MARINE ANIMALS 



consisting of fibres which penetrate the pigmented wall of the photophore 

 and ramify among the photogenic cells. 



In the luminescent ophiuroids, such as Amphiura, Ophiopsila and 

 Ophiothrix, light appears on the arms, less often on the body, and is 

 localized in spines, plates or tube feet, according to the species. The struc- 

 tures responsible are unicellular photogenic glands, possessing long ducts 

 which open to the exterior. No extracellular luminous secretion has been 



Reflecto 



; $ht 

 body 



Pigment 

 sheath 



Light body 



Fig. 13.13. (a). A Simple Light Organ of a Cranchiid Squid Liocranchia 



valdiviae 



The photophore consists of slightly invaginated surface epithelium, backed by a 

 connective tissue reflector layer. (After Chun, 1910, and Dahlgren, 1916.) 



(b) Integumentary Photophore of Calliteuthis reversa, a Bathypelagic Squid. 



(After Chun, 1910.) 



detected in these animals, however, and it seems that the light is intra- 

 cellular in origin. 



The luminescence of Pyrosoma and Salpa has been ascribed to symbiotic 

 bacteria (11), but according to Harvey (28) the evidence favours inherent 

 luminous ability on the part of the animal. In Pyrosoma each individual of 

 the colony lightens, and the light appears in two groups of test cells lying 

 at the entrance to the branchial cavity in the peripharyngeal blood spaces 

 (Fig. 13.15). These cells contain curved cytoplasmic rods which may be 

 responsible for producing the light. 



Segmentation and young embryonic stages of Pyrosoma are also lumi- 

 nescent due to the participation of test cells in the developmental cycle. 

 These test cells migrate into the developing embryo and arrange them- 

 selves about the germinal disc. When the four primary ascidiozooids arise 



