CONTRIBUTIONS TO THE ANATOMY OP THE HIRDDINEA. 435 



run into this layer terminating just underneath the epi- 

 dermic layer (see figs. 9 — 14, m. rad.). The contractile 

 blood-spaces which are developed in the lateral appendages 

 in Branchellion and the rudimentary organs of the same 

 nature which exist in Piscicola, Pontobdella, and Clep- 

 sine (see infra, Circulatory System), lie in this layer. 



All the connective and vasifactive tissue elements exist 

 merely as packing to the mucous glands of the epidermis, and 

 in forms where they are excessively developed (e. g. Hse ma- 

 dips a, the great development being probably connected with 

 its terrestrial mode of life) there is room for comparatively little 

 connective tissue. 



iv. Muscles — Proboscis. 



I do not propose to enter here at any length into the arrange- 

 ment of the body muscles, the older anatomists, Thomas, Spix, 

 Brandt, Duges, and Moquin-Tandon, with their marvellously 

 patient dissections, have indeed left little to be added to our 

 knowledge in this respect ; while more recently Leydig, Leu- 

 ckart, Vaillant, and Lang have reviewed and added to this 

 knowledge in respect of individual genera. 



Muscles of the Body Wall. — As in all vermiform animals 

 depending for their power of locomotion upon the muscles of 

 the body wall alone, these muscles^re arranged in circular and 

 longitudinal series. The most external layer (figs. 9 — 14, 

 61 — 64, m. circ.) is arranged directly transverse to the long 

 axis of the body. Within this layer there may be a series of 

 fibres which have a diagonal course (1. c. m. obi.), and within 

 these lies the main mass of longitudinal fibres (1. c. m. long.). 



There is considerable variation with regard to the arrange- 

 ment of these fibres in diff'erent genera (see figs. 9 — 14, 61 — 64), 

 Clepsine presenting the simplest arrangement and Hirudo 

 the most complicated ; in the latter, connective tissue with 

 embedded longitudinal fibres being inserted between the 

 circular and diagonal layers (fig. 13). 



Dorso-ventral andRadialMuscles. — These fibres belong 

 to one system, the more centrally placed at the sides of the 



