354 



Mr. A. G. Bourne. 



[June 21,. 



Rounded connective tissue- cells are rare among the Gnathobdel- 

 lidee, Trocheta being the only genus which presents such. They 

 occur in masses, and are also very generally arranged in rows, pro- 

 bably prior to their conversion into botryoidal tissue. 



2. Ectoplastic metamorphosis — the cells forming fibres. 



The cells of most wide-spread occurrence are cells which have elon- 

 gated, and it may be branched and formed fibres. 



It is possible to trace this process ; a slightly irregular cell elon- 

 gates more and more, and its processes become drawn out, so that 

 ultimately little cell-substance is left and a long very fine fibre is- 

 produced, the cell all the time doubtless adding to matrix. 



In Pontobdella these fibres may become elastic. They always run- 

 singly. 



3. Ect-entoplastic metamorphosis — the cell developes pigment. 

 a. The cells take no part in the formation of a vascular system. 

 This series of modifications is well seen in Pontobdella. Indifferent 



cells develope pigment ; this may be traced, originating in young 

 animals (recently hatched). 



Such cells either remain rounded or they divide into irregular 

 groups, and afterwards become much branched. The process may 

 be shortened, the cell branching, and forming pigment simultaneously. 



The rounded cells lie more deeply, the branched cells lie more 

 superficially, and form the pigment of the dermis. 



/3. The cells take part in the formation of a vascular system — 

 Botryoidal tissue — " Yasofibrous " tissue. 



A set of modifications similar to those just described takes place, 

 but intracellular vacuolation taking place at the same time vascular 

 spaces are formed. These come in communication with the capillaries 

 of the true vascular system on the one hand, and with the sinuses on 

 the other. 



4. Entoplastic metamorphosis— Vacuolation to form Capillaries. 

 The capillaries of the true vascular system are probably formed by 



the vacuolation of indifferent connective tissue cells. 



It may be noticed here that in forms where no canalisation of pig- 

 mented cells has occurred, the blood is always colourless, while in 

 forms with red blood such canalisation of pigmented tissue has 

 occurred in the formation of the vascular system. 



Blood and Blood Spaces. 



Blood. — In the Rhyncobdellidae — 

 The blood is colourless. 



Colourless amoeboid corpuscles occur in very large numbers, but 

 present no remarkable histological characters. 

 In the Gnathpbdellidae — 



The blood is red, the plasma containing dissolved hemoglobin. 



