384 SUMMARY OP CURRENT RESEARCHES RELATING TO 



very well marked, and in some of tlie tubes there are indications of 

 septa or of a meshwork. Two nerves enter the carapace ; the anterior 

 is directed towards the head, and the other, which is thicker, is distri- 

 buted to the hinder portion. With repeated divisions these nerves 

 become smaller and smaller and very difficult to follow out ; they lose 

 themselves in the cellular matrix-layer of the integument. The mode 

 of termination of the nerves in the setae is not what has been ordinarily 

 supposed. In the setse at the edge of the carapace the inner filament 

 appears as a hard line or looks as if it were cuticularized ; at its root, in 

 Argulus phoxini, a small corpuscle with similar optical characters may be 

 seen. Prof. Leydig formerly regarded these structures as being nervous in 

 nature, but he is now convinced that they are connected with the sup- 

 porting or skeletal tissue which is distributed between the two plates of 

 the shield. The nerves lose themselves in the cellular substance of the 

 matrix layer. A connection between the internal filament of the seta 

 and the nerves is only to be regarded as due to the fact that the 

 hyaloplasm of the nerves may pass into the substance of the layer and 

 thence flow over into the setas. 



In the frontal eye the most striking character is the pigment ; there 

 are scattered, yellow, fat drops, a diffused blue pigment, a pigment 

 consisting of molecular granules, and a brownish pigment which is so 

 arranged as to form cup-like divisions ; of these last there are four. In 

 the paired eye there' is a homogeneous matrix-layer which gives rise to 

 the cuticle ; the membrane is a continuation of the neurilemma of the 

 optic ganglion, and corresponds to the connective tissue which surrounds 

 the eye of higher animals. The crystalline cone is surrounded by a 

 special envelope which clearly corresponds to the tube which incloses 

 every cone and its nerve-rod in the compound eye of other Arthropods. 

 There is a quadrangular area at the hinder part of the lens, and with 

 this there is connected a nerve-rod, which is likewise four-sided. The 

 crystalline cones exhibit a distinct and remarkable dimorphism ; there 

 are smaller cones which form a special compact group near the hinder 

 margin of the eye ; they are not only distinctly four-lobed but surrounded 

 by a darker margin than the rest ; there are about a dozen of them. 

 The pigment of the eye is dark-violet in parts, and elsewhere brown. 



The body-cavity is stated to be at first a blood or lymph-space ; 

 canal-like constrictions and ramifying prolongations become blood-lymph 

 vessels; the spaces and canals are bounded by matrix-cells of the 

 cuticular and connective tissue which secrete on the inner surface a 

 homogeneous fringe ; there is as close a connection between connective 

 tissue and blood-spaces as between hill and valley ; the final processes 

 of the cavitary systems are the ducts in the clefts of the connective 

 tissue and the pore-ducts of the cuticular. 



There is a fluctuation rather than a circulation of the blood. The 

 heart is a median cylindrical tube which is, primitively, a longitudinal 

 cleft between the muscles of the back, and so completely resembles other 

 blood-spaces found between muscles. Even in the adult stage the anterior 

 end of the heart has no definite limits. Its histological structure is 

 difficult to make out ; the inner bounding line is not perfectly regular, 

 but appears to be alternately thickened and narrowed. In animals which 

 have been for a long time without food the blood-corpuscles become 

 rounded, and one large or several smaller vacuoles appear in their 

 interior. 



