1 3 PATTEN AND REDENBAUGH. [Vol. XVI. 



the most anterior visceral arches are likewise devoid of gill 

 lamellae. On the neural side of each thoracic appendage of 

 LimuliiSy except the first and last, is a large group of sense 

 organs supplied by a special ganglionated tegumentary nerve, 

 i.e., the gustatory organs of the coxal spines (Patten, '93) and 

 the sense organs on the endopodite of the abdominal append- 

 ages. These sense organs and nerves correspond in position 

 and in mode of development respectively to the suprabranchial 

 sense organs and the rami dorsali of the cranial nerves of 

 vertebrates. 



IV. The Dermal Skeleton. — Lhnnlus is the only invertebrate 

 where the chitinous exoskeleton has begun to form a system 

 of true dermal bones (Patten, '94). They arise as innumerable 

 ingrowths of the ectoderm that unite to form a mass of anasto- 

 mosing chitinous trabeculae. The tissue thus formed resem- 

 bles coarse cancellated bone, but more especially the coarse bony 

 networks that form the inner layers of the cephalic shields of 

 the Cephalaspidae. Within these trabeculae are numerous cavi- 

 ties, which, after the shell is macerated and dried, become filled 

 with air, and then bear a strong resemblance to true bone 

 lacunae. They are spindle-shaped lacunae, with two or more 

 very fine canal iculi leading off from them, which appear to unite 

 in some cases with the canaliculi of neighboring lacunae. In 

 living tissue the lacunae are filled with a substance resembling 

 protoplasm, and the larger ones appear to be nucleated. Thus, 

 an entirely new dermal structure is forming here unlike that 

 known in any other invertebrate ; namely, local ingrowths of 

 the ectoderm, forming a network of chitinous trabeculae, 

 into which numerous cells migrate to form true bone cor- 

 puscles. 



As the trabeculae cannot be shed periodically, like the rest 

 of the exoskeleton, they are retained permanently within the 

 body. Since, as we now know, some forms of chitin are very 

 closely allied to chondrin, perhaps this condition may be the 

 means of ultimately completing the chemical metamorphosis 

 of chitin into chondrin. But there is no evidence that other 

 chitinous ingrowths, such as the entopophyses or the apodemes 

 of many arthropods, are invaded by cells, or that they have 



