354 PALAEOSTRACA. 



the limbs, but connected together anteriorly and posteriorly. The 

 space within the limb-rudiments is completely filled with mesoderm- 

 cells. Paired, segmented coelomic cavities soon appear, as in the 

 Arachnids, and are continued into the limb-rudiments. Many small 

 spaces arise between the mesoderm-cells, and these unite to form the 

 coelomic cavities. The enlargement of these cavities divides the 

 mesoderm into a splanchnic and a somatic layer. Laterally, how- 

 ever, these two layers pass into one another, and this single layer 

 grows up dorsally ; this dorsal continuation only splits into a somatic 

 and a splanchnic layer at a later stage, after the appearance of the 

 heart. A paired, longitudinal thickening develops very soon in the 

 single dorsal mesoderm-layer ; this is the rudiment of the dorsal 

 longitudinal muscle, and, at the same time, of the points of insertion 

 of the limb-muscles which run up from the ventral side. These 

 latter muscles develop in the mesodermal septa which grow in from 

 the ventral and lateral surfaces, and divide the mass of food-yolk 

 into a number of lobes (originally into six) ; this gives rise to an 

 apparent segmentation of the internal organs of the cephalo-thorax. 



As soon as the mesodermal plates meet in the middle dorsal line, 

 a longitudinal thickening, the rudiment of the heart, arises at their 

 point of junction, KINGSLEY was unable to decide whether the 

 cells which combined to form this thickening belonged exclusively 

 to the mesoderm, or whether they should be considered as immi- 

 grating yolk-cells. A lumen soon appears within the rudiment of 

 the heart, into which a few cells which have become detached from 

 the walls wander, and these change later into blood-corpuscles. The 

 wall of the tubular heart now separates from the splanchnic layer, 

 but only later from the somatic layer of the mesoderm. 



In later stages of development, the coelom undergoes degeneration, 

 all the spaces of the body-cavity becoming traversed by reticular 

 connective tissue. These processes have, however, not yet been 

 accurately described. 



Limulus, like the Arachnids, is distinguished by the possession 

 of an inner skeletal body, lying between the ventral chain of ganglia 

 and the alimentary canal, and consisting of tissue resembling fibro- 

 cartilage. This is the endosternum (Fig. 163, ent, and p. 356), which 

 serves for the attachment of many groups of muscles. According 

 to BROOKS and BRUCE (No. 1 0), the endosternum arises as a thicken- 

 ing of the splanchnic mesoderm on the ventral side of the mass 

 of food-yolk (rudiment of the enteron). 



PACKARD'S " brick-red gland," which we agree with RAY LANKESTER 



