142 PROFKSSOR E. EAY LANKESTER. 



large proportion of the protoplasmic cellular element ; (2) the 

 disposition of the cells around spaces or lacunse which are 

 of an oval or polygonal character, and communicate freely 

 with one another. We may conceive of the conversion of the 

 fibroid skeletal tissue into lacunar tissue by supposing the 

 fissures in the skeletal substance iti which the cells are placed 

 to become dilated and widened at the expense of the skeletal 

 substance itself, which would be so reduced by this process as 

 to become a mere membrane separating the various fissures 

 from one another, and even breaking down altogether at 

 numerous points so as to allow one enlarged fissure or lacuna 

 to communicate with another. Then further we should make 

 the cells adhere closely to the wall of the enlarged space in 

 which they exist, projecting irregularly in an amoeboid fashion 

 into that space, their protoplasm being more abundant than 

 was the case in the confined cell-groups of the fibro-massive 

 skeletal tissue. 



In Limulus the cells of the lacunar tissue lying between the 

 cseca of the gastric gland contain usually each a single spherical 

 drop of a highly refringent substance of a yellowish brown 

 colour, which is of a fatty nature, though not readily soluble 

 (PI. XI, fig. Id). In Scorpio numerous smaller granules 

 are scattered in the protoplasm of the corresponding cells, 

 resembling the granules which occur also in the blood- 

 corpuscles (PL XI, fig. 3 d). 



The formation of the lacunar tissue is so regular as to give 

 the appearance in section of a number of polygonal areolae 

 fitted side by side, as though the section had traversed a 

 mass of closely packed capsules. We may therefore apply the 

 term * capsule ' to the skeletal substance {b in the figures) 

 which forms the firm boundary of each of the polygonal 

 spaces whilst the term Macuna' applies to the space (e in the 

 figures) left centrally between the cells lining the capsule. 



The membranous skeletal substance which forms the capsules 

 appears to be chitinous in nature like the matrix of the 

 fibroid tissue of the entosternite. After maceration it may be 

 obtained (in the case of Limulus) free from the protoplasmic 



