312 ENDOCRANIUM, BRANCHIAL AND NEURAL CARTILAGES. 



for example are well developed at a very early embryonic period, and appear 

 to be quite out of proportion to the volume and functional importance of the 

 surrounding muscles. Moreover isolated and highly developed cartilage cells, 

 or cell nest, may be seen in adult crabs lodged in the perichondrium of the gill 

 bars, or at the ends of the entopophyses and in other places where they appear 

 quite foreign to the surrounding tissues, and with nothing to suggest the reason 

 for their presence in such unusual surroundings. 



IV. The Endocranidm. 



The endocranium, variously named prosomatic endosternite, cartilaginous 

 sternum, or plastron, has been found in many of the arachnids and in the 

 phyllopods. In 1889, I figured and described the endocranium of the scorpion, 

 and compared the endocrania of the scorpion and Limulus with the primordial 

 cranium of vertebrates. In 1899, in collaboration with Mr. Redenbaugh, a 

 graduate student in Dartmouth College, the endocrania of Apus, Mygale, and 

 Limulus were described and illustrated in more detail. 



The endocranium of arachnids is a broad plate of fibro-cartilage lying on 

 the haemal side of the brain. It serves primarily for the attachment of the 

 muscles that move the oral appendages, and for the flexor muscles that move the 

 cephalo-thorax on the branchial section of the body. In scorpions and in Limulus 

 a complete occipital ring is formed about the spinal cord, near its union with 

 the brain. 



The floor of the endocranium is a continuous structure and there are no 

 indications that it consists of originally separate pieces; the supra-occipital plate, 

 when present, may be regarded as a modified neural arch belonging to the 

 vagus segments. We recognize in the higher arachnids three principal parts, viz. 

 two lateral bars; a broad median plate that unites the posterior ends of the 

 bars; and a bridge of cartilage on the neural side of the nervous system, which 

 together with the above mentioned parts forms a closed ring about the anterior 

 end of the spinal cord. 



The endocranium has a true cartilaginous consistency, and is composed of a 

 mass of interwoven fibers and a dense matrix containing stellate lacunae united 

 by anastomosing canaliculi. In the living cartilage, the canaliculi contain 

 minute branching processes of the cartilage cells situated in the lacunas. 



The endocranium of arachnids, as I first pointed out in 1889, represents 

 the ancestral stage in the evolution of the primordial cranium of vertebrates, the 

 lateral bars, the transverse plate, and the neural arch of the arachnid cranium 

 corresponding respectively to the trabeculse, the parachordals, and the occipital 

 ring of the vertebrates. 



The Endocranitim of Apus. (Fig. 212.)— In Apus the endocranium 



