20 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



undergo various stages of contraction or shortening. In extreme 

 contraction, the entire animal appears (A') like a small, rounded, 

 gelatinous knob, brown or green, according to the species. The 

 external appearance presents certain characteristic features in 

 connection with reproduction (see below). 



By looking down upon the distal end of Hydra, it is seen that 

 a number of radiating lines, all passing through similar parts, can 

 be drawn from the centre of the mouth along the tentacles. This 

 sort of symmetry, common among lower animals, is of the kind 

 termed radial. 



2. Body-wall. Amoeba, Gregarina, and Vorticella are unicellular, 

 each being made up of one cell or morphological unit. These animals 

 therefore possess diverse parts in virtue of the specialization, 

 differentiation, of the protoplasm of single cells. Hydra, on the 

 other hand, is multicellular, being made up of very numerous cells, 

 each of which is morphologically equivalent to an Amoeba. These 

 cells, however, are modified in various ways for the performance 

 of diverse functions. This is the principle of physiological division 

 of labour, arid the accompanying diversity of form is termed 

 morphological differentiation. Aggregates of cells, similar in 

 form and mode of origin, and specially capable of carrying on a 

 particular function or functions (instead of all functions, as in 

 Amoeba), are known as tissues, and these again are interwoven 

 into organs, digestive, reproductive, &c. In Hydra this special- 

 ization is not very complete ; but, as the animal scale is ascended, 

 physiological division of labour and morphological differentiation 

 become more and more marked. 



Histology. The body of Hydra contains a large digestive cavity, 

 prolonged into the hollow tentacles. The wall of this cavity is 

 the body-wall, and the cells composing it are divided into two 

 distinct layers, an external ectoderm (ec), and an internal endoderm 

 (en), about twice as thick. Between the two is a very thin 

 structureless membrane, the mesoglcea (intermediate or supporting 

 lamella) (i.F), not composed of cells. 



The ectoderm (ec) is mainly made up of large somewhat conical 

 cells (C) which are broadest externally, while the narrow internal 

 end of each of them is drawn out into one or more contractile 

 tail-like processes, which may branch. These "tails" take a 

 longitudinal direction, and are closely attached to the mesogloea. 

 Each large ectoderm cell possesses a clear external border, and 



