GLOSSARY 495 



Triplostichous (Gr. triploos, threefold ; stichos, layer) : an eye consisting of 

 three layers : preretinal, retinal, and postretinal, e.g. the median eyes 

 of the larval scorpion, in which the retina is inverted. The post-retinal 

 layer may be pigmented or serve as a reflecting membrane (tapetum 

 lucidum or argentea). 

 Triton : a marine gastropod mollusc enclosed in a univalve spiral shell, and 

 possessing two simple eyes of the upright type situated near the bases 

 of the tentacles. 

 Trochelminthes (Gr. trochos, wheel ; helmins, worm) : an invertebrate 

 phylum comprising the wheel animalcules or rotifers. The free-swim- 

 ming larva is known as a trochosphere or trochophore. 

 Trochophore (Gr. trochos, wheel ; phoreo, bear or carry) : the cylindrical, 

 free-swimming larva of certain annelids and molluscs. It is characterized 

 by two or more circlets of cilia, resembling those of the rotifers, and the 

 presence of an apical plate bearing an upright tuft of cilia, and often 

 two ocelli. 

 Tunicata (L. tunica, cloak) : a subphylum Urochordata, which includes the 

 ascidians or sea-squirts, so named on account of the thick, leathery coat 

 with which they are covered. 

 Turbellaria (L. turbo, disturb) : a class of flat worms, mostly leaf-like in form, 

 including the Planaria. Pigmented eye-spots are found on the dorsal 

 aspect of the head-region. 

 Urochorda (Gr. our a, tail ; chorde, string) : a subphylum of the Chordata, in 

 which a notochord limited to the tail region is present in the free- 

 swimming larva. 

 Urodela (Gr. oura, tail ; delos, visible) : tailed amphibians, including the 



newts and Proteus anguineus. 

 Varanus : a lizard of the Monitor family in which the parietal sense-organ is 



large and well differentiated. 

 Veliger (L. velum, a sail ; gero, carry) : a stage in the development of certain 

 Gasteropods, e.g. Vermetus. The preoral circlet of the trochophore 

 larva has become bilobed, and by means of its cilia serves as an organ 

 of locomotion. Paired lateral eyes are present at the bases of the tentacles. 

 The term is also applied to one stage in the development of Geotria. 

 Velum transversum (L. velum, veil or curtain) : the thin fold of the roof of 

 the third ventricle, which forms the anterior boundary of the dorsal 

 sac. Laterally it is continuous with the choroid plexuses of the lateral 

 ventricles. 

 Yungia : a marine Planarian belonging to the order Polycladidas, class Turbel- 

 laria, of the flat- worms. Ocelli are developed over the region of the 

 apical cells of the free-swimming larva. 

 Zcea (Gr. zoon, living being, animal) : a larval stage of the crab, characterized 

 by the very large size of the compound eyes as compared with the size 

 of the body. 



