SKELETO-TEOPHIC TISSUES AND COXAL GLANDS. 141 



common in all larger Arthropoda have a totally different and 

 thoroughly characteristic structure. 



Having found this small but undeniable true internal skele- 

 ton in one Crustacean, I have been anxious to look for it in 

 others. At present I have only examined hastily some Deca- 

 poda, and I find that there (Palsemon, Astacus) the incom- 

 plete archway formed by the thoracic epidermal apodemata is 

 completed anteriorly in the median line by a small piece of fibro- 

 skeletal tissue similar in histological character to that forming 

 the entosternite of Arachnida. It is, however, extremely small, 

 and obviously the development of floating skeletal plates 

 formed of dense connective tissue does not take a prominent 

 place in the economy of the higher Crustacea as it does in that 

 of the Arachnida. It is remarkable that the most archaic of 

 living Crustacea (Apus) should possess a well-marked though 

 small entosternite of mesoblastic origin. 



Probably enough such entosternites of dense connective tissue 

 will be found, when properly looked for, in not a few other 

 Arthropoda. 



N.B. — In all the above cases the tissue of the entosternite is 

 non-vascular. 



3. The lacunar connective tissue of Limulus and Scorpio; 

 pigmentiferous tissue and the finer blood-vessels. 



The most abundant and widely-distributed variety of con- 

 nective tissue in Limulus and the Scorpions is that which I pro- 

 pose to term " lacunar connective tissue." This tissue "■ packs " 

 the caeca of the gastric gland and genital ducts, and is also 

 found between the bundles of muscular tissue. It passes over 

 at certain points into membranous connective tissue, and again 

 in the neighbourhood of the entosternite is suddenly trans- 

 formed into the fibro-massive skeletal tissue of that organ. 



The lacunar connective has a very definite character which 

 is shown in Plate X, figs. 1 and 2, and in Plate XI, figs. 1, 2, 3. 

 The distinctive features about this tissue as compared with the 

 fibro-massive skeletal tissue which we have noticed above is 

 (1) the very small amount of skeletal matrix and the relative 



