ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 611 



organs do not appear to have given rise to the non-sogmental organs 

 which are limited to a specialized part, and have arisen in response to 

 the increased needs of their possessor. In the land-leech it is possible 

 that, in addition to distinguishing between light and darkness, the 

 segmental organs have some olfactory function, but this does not 

 appear to hold good for Macrobdella. 



Metamorphosis of Aulostoma gulo.* — Dr. R. S. Bergh describes 

 the larva of Aulostoma gulo as varying in size, and having an oval 

 form of body in which the body-wall and enteric wall are widely 

 separated from one another; the oesophagus lies on the ventral 

 surface, and may be single or divisible into pharynx and oesophagus 

 proper ; behind the mantle are the stripe-like fused trunk-rudiments, 

 and at the sides of these the four pairs of circular primitive kidneys ; 

 all the structures of the larva fall into two distinct categories, those 

 which have already specific functions, and those which are still 

 indifferent cell-masses. To the former group there belong the 

 primitive epidermis and the subjacent muscular and nervous cells 

 ■ of the body-wall, the enteric canal, and the primitive kidneys ; to 

 the latter the head and trunk-germs. 



The primitive ectoderm is a simple flattened epithelium, the 

 boundaries of the constituent cells of which are not apparent ; the 

 musculature consists of two different kinds of smooth elements ; some 

 are small and closely packed, the others are large and do not form a 

 true muscular layer, their cells being separated from one another by 

 rather broad intermediate spaces ; they are, as a rule, arranged 

 transversely, and so appear to form the circular muscle of the body. 

 The excellent description given by Leuckart of the muscular 

 system of the larva of the medicinal leech agrees essentially with 

 that of Aulostoma. Cells which are apparently nervous in nature are 

 to be found scattered between the muscle-fibres ; they are spindle- 

 shaped or much branched, and their process os are often exceedingly 

 long and fine. The enteric canal is divisible into an anterior 

 oesophageal portion and a midgut, which ends blindly ; the former 

 has a surprisingly complicated structure, for it consists of four 

 distinct layers : an epithelial without distinct cell-boundaries, a layer 

 of circular and then a layer of radial muscular cells, and an outer 

 layer of epithelium. The midgut has the form of a simple sack 

 which occupies by far the greater part of the cavity of the body ; its 

 walls are simple in structure. 



The four pairs of primitive kidneys, which are ventral in position, 

 are circular closed organs, formed of two rows of cells ; in the 

 anterior pairs a canalicular structure can be easily made out, and 

 here and there there are anastomosing tubular spaces ; the cell- 

 boundaries, however, are indistinct, and the cells elongated. 



The head-germs lie in front of the oesophagus, between it and 

 the epidermis, and have, at an early stage, a broad tri- or quadri- 

 lobate form ; they early unite with one another, as do also (in contra- 

 distinction to Nephelis) the trunk-germs ; these latter are much better 



* Arbeit. Zool.-Zoot. Inst. Wurzburg, vii. (188G) pp. 231-91 (4 pis.). 



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