PROFESSOR OWEN ON LIMULUS POLYPHEMUS. 207 



middle of the series ; the hindmost again diminished, but in a less degree ; the whole 

 mass has the full oblong or irregular oval form shown in fig. 2, PI. XXXVI. Each 

 plate is strengthened by a chitinous filament along its free border, thickest where this is 

 exposed, so that the length of the gill is greater at its free or floating side than along its 

 attached base : the free margin is also ciliate. 



" Each gill-plate consists or two layers or membranes, united along the chitinous 

 border, and also by numerous filaments, so far apart as to divide the interspace into 

 reticular canals or cells, smallest at a subcentral space (fig. 3 a), and affecting a con- 

 centric arrangement as they approach the free borders of the gill-plate. The two 

 constituent layers of the branchial plate may be regarded as productions or duplicatures 

 of the delicate skin of the upper or inner surface of the lamelliform limb. 



" From a venous sinus along the base of attachment of the gill-plates 1 the blood passes 

 freely into the interlamellar spaces, whence it enters the vessels coursing along the border 

 of each plate, from the inner side, where the vessel is largest, towards the outer side 

 (PI. XXXVI, fig. 2 a). Here appears to begin the returning system of branchial veins 

 on the fore part of the base of attachment. These veins ascend and converge on each 

 side of the intestine, and traverse the pericardial sinus to enter directly the heart by the 

 five pairs of ostia at the widest posterior part of that organ. 



" The muscles which divaricate the branchigerous limbs, and at the same time 

 separate the gill-plates and expand their cavities to the extent permitted by the interposed 

 columns, act as inspiratory ones, inviting the flow of blood from the abdominal sinuses 

 into the cavities of the gill-plates. This action may be supposed to take place when the 

 King-crab is moving or resting in its atmosphere of sea-water. The muscles which 

 approximate the branchigerous plates and press them against each other and the 

 thoracetron will close the ciliate slits leading to the gills, will compress those organs, 

 and tend to squeeze the blood from the reticulate interspace of their constituent lamellse. 

 Such movement must be ' expiratory ' and also effective in defending the delicate 

 surfaces of the branchial membranes from the atmosphere of muddy or sandy sea-water 

 when the King-crab is burrowing either for food or concealment. 



§ 6. — " Generative System. — In Limulus the sexes are distinct; 3 the male is smaller 

 than the female, and in both the generative organs lack those accessory parts that relate 

 to intromission in some higher Crustaceans. 



" The ovarium is a system of ramified tubes and cavities, occupying chiefly the dorsal 

 region of the body ; it extends along the median part of the thoracetron, and expands 



1 " ' II parait exister une libre communication entre ces diverses poches respiratoires ; car, en 

 introduisant de l'air dans une de ces duplicatures on voit non seulement s'ecarter les lames de la meme 

 branchie, mais meme se gonfler toutes les branchies, ainsi que l'espace membraneux entre les pattes 

 abdominales,' J. van der Hoeven, op. cit, p. 19. The intercommunicating passage is the basal sinus, 

 related to the gills, physiologically, as a ' branchial artery.' " 



2 [The separation of the sexes characterises the Crustacea generally, save in the aberrant Cirripedia, in 

 which the greater part appear to be hermaphrodites.] 



