296 GLOSSARY. 



Tympanum. The drum of the ear. In insects certain membra- 

 nous parts subservient to sound. 



Typhlosome. A peculiar fold of the intestine in the Tunicata and 

 the Lamellibranchiata. 



Typical. What is the most representative of a group. 



Ulna. One of the two bones of the forearm. 



Ulotrichous. Having woolly hair; the hair is also flattened, and 



its section oval. 



Umbo. The boss or beak near the hinge of a bivalve shell. 

 Umbrella. "The gelatinous bell of a medusiform planoblast" 



(Allmaii). " A swimming-bell with the velum" (id.). Without 



a velum (Huxley}. 



Unguligrade. Walking on the hoofs. 

 Uropoda. The three pairs of expanded hind legs in certain 



Crustacea. 

 Uropoietic system. The organs for the secretion of water and 



urea ; in the invertebrates they are supposed to be represented 



by the water-vascular system. 



Urosthenic. When the maximum of strength is in the tail. 

 Urostyle. A prolongation of the spinal cord in certain fishes 



and amphibians. 

 Uterus. The womb. 



Vacuoles. Certain cavities in the Rhizopoda having a contractile 

 or rhythmical movement. 



Variability. All individuals vary, but accidental variability is 

 due to "indeterminate antecedents," and only exists to a " very 

 small extent," and only in plastic forms. Some species have a 

 " singularly inflexible organization" (Darwin). 



Variety. Any departure from the parental type. 



Veliger. An advanced larval form of Mollusca when the velum 

 is fully developed. 



Velum. (1) In a medusiform gonophore, "the membranous 

 perforated diaphragm which stretches transversely across the 

 codonostome " (Allmari). (2) A ciliary expansion of the integu- 

 ment attached to the head in the larval Mollusca. 



Veiia cava. The great vein that returns the blood to the heart. 



Ventricle. A cavity ; one or two in the heart, two in the brain. 



Vertebra. The bones forming the spinal column. 



