894 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Branchellion and the extreme form Hirudo are next discussed ; the 

 first and last are almost identical in their external characters. 



The cells of the epidermis may become glandular or sensory ; the 

 former are either superficial and mucous, or more deeply seated, when 

 they may be salivary, clitellar, or prostomial ; the functions of these 

 last stand in need of further investigation. The muscular system is 

 next described, and its cells stated to be very long, and in some cases 

 much branched ; there is a cortical layer which, in transverse section, 

 is seen to exhibit longitudinal fibrillation, and a granular medullary 

 substance with a large oval nucleus. The connective substance difiers 

 in the extent of its development in different genera, and the amount 

 of it is in direct proportion to the " limpness " of the leech. The 

 cells of which it consists undergo en to- or ecto-plastic metamor- 

 phosis ; in the former case the cell retains its rounded form, and we 

 may have vacuolated cells, or fat-cells ; the most common repre- 

 sentatives of the latter are the elongated or branched corpuscles, 

 which can be easily studied in Hirudo, though best in Pontohdella. 

 A third case is called that of ect-ento-plastic metamorphosis, and 

 here the cell developes pigment ; in the simpler conditions the cells 

 take no part in the formation of a vascular system (Rhyncobdellidas) ; 

 in the Gnathobdellid^ the cells take part in the formation of a 

 vascular system, botryoidal tissue, vaso-fibrous tissue ; the mode of 

 development is best studied in Aulostomum. Vacuolation to form 

 capillaries is a mode of entoplastic metamorphosis. It is found that 

 all the forms of connective and vasifactive tissue may be derived from 

 an indifferent connective-tissue corpuscle. The phenomena they exhibit 

 lead on to the general question. Is there any well-founded distinction 

 to be drawn between spaces in the animal body with regard to their 

 relations to the cell or cells surrounding them ? 



Mr. Bourne points out that some of the spaces in the animal body, 

 for example, the contractile vacuoles of Protozoa, the ducts in the 

 nephridial cells of leeches, and so on, are obviously formed by actual 

 metamorphosis of the cells themselves, and are to be contrasted with 

 such spaces as the lumina of invaginated gastrulse, which are formed 

 outside cells ; we may, then, distinguish between endocytic and para- 

 cytic coelosis. Both these processes of lumen formation (coelosis) may 

 be direct, the lumen appearing at once, or indirect, the appearance of 

 the lumen being delayed. 



The single vascular fluid of the Hirudinea corresponds to both 

 coelomic and red vascular fluids as found in the Chaetopoda. In the 

 Ehyncobdellidae the blood is colourless, in the Gnathobdellidae it is 

 red, the plasma containing dissolved haemoglobin. The author enters 

 in great detail into the characters of the ccelomic spaces, and prefaces 

 it by saying that by the word " coelom " he understands " a space or 

 set of spaces excavated in the mesoblast and distinct from blood- 

 vessels, such as is the body-cavity of Chsetopoda and Vertebrates," 

 and he does not "undertake to discuss whether such space is a 

 pseudocoel or an enterocoel in the Hertwigs' sense, or may be some- 

 thing altogether unprovided for in the artificial and valueless system 

 of those authors." The Gnathobdellidae are to be distinguished from 



