12 PATTEN AND REDENBAUGH. [Vol. XVI. 



in the thoracic region, belong primarily to the category of 

 abdominal appendages, as shown (i) by the development of 

 the chilarial neuromere ; (2) by the distribution of the haemal 

 nerves of the neuromere ; and (3) by the possession of a gill 

 bar of capsuliginous cartilage closely resembling those found 

 in the other abdominal appendages. 



The opercular nerves {n.n.^, Figs. 3 and 4) arise a little poste- 

 rior to the chilarial nerves and pass backward alongside the 

 ventral cord through the occipital ring and innervate the genital 

 operculum. The haemal nerves (Ji.n.^, Figs. 3 and 4) belonging 

 to this neuromere arise slightly posterior to the haemal nerves 

 of the chilarial neuromere, and, diverging at a less angle than 

 the preceding, pass through the occipital ring and out towards 

 the sides of the body immediately posterior to the capsuligi- 

 nous bars {cap.b). As they turn outwards each sends a branch 

 {in.n.^, Figs. 3 and 4) haemally to the intestine and longitudinal 

 abdominal muscles. The main part of the nerve finally sends 

 a branch to the heart, and then distributes itself over the 

 surface of the abdomen. The ventral cord {v.c, Figs. 3 and 4) 

 passes straight back from the oesophageal collar through the 

 occipital ring and does not branch until it reaches the ganglion 

 of the first gill metamere. 



II. Abdominal Endochondrites of Limulus. 



In the abdominal region of Lhnuhis is a series of cartilages 

 spanning the ventral cord on the neural side. There are six in 

 all, one at the base of each of the abdominal appendages, from 

 the operculum to the fifth gill. Like the plastron, they are 

 composed of fibroid cartilage and serve as centra for the 

 attachment of muscles, but they differ from the plastron in 

 being placed on the neural side of the central nervous system 

 instead of upon the haemal side. These cartilages vary consid- 

 erably in different individuals and in different metameres of the 

 same individual, but the one represented in Fig. 3 (ab.en.) is 

 typical. It consists of an irregularly shaped body with a flat 

 neural surface, which is in contact with the integument except 

 across the middle portion. This middle part lies directly under 



