THE EXDOSKELETOX 713 



patella, or knee-cap bone, is analogous to the olecranon process, though 

 it never joins the other bones. 



The ankle joint in mammals is never intratarsal, but always between 

 tarsal and crural bones. 



Where the bones of the foot rest on the ground as in man and in 

 the bear, such a foot is known as plantigrade. Where the toe of the 

 foot includes only the distal phalanges such as in the dog and the cat, 



comes down again toward the sea in autumn ; and the Planarian Lamprey, 

 (Petromyzon planari) , from 20 to 30 cm. long which inhabits the calm waters 

 entirely, and is commonly found in rivers. 



Fig. 1. A lamprey (Petromyzon planari) with its mouth fixed to a rock. Pg., 

 Genital papillae. 



Fig. 2. Anterior part of the body of Petromyzon marinus, showing the seven 

 branchial openings and the buccal cupping glass surrounded by little papillae. 

 The olfactory opening lies in front of the eye. 



Fig. 3. Section through the anterior region of Petromyzon marinus. The section, 

 slightly oblique, is nearly sagittal toward the front ; it deviates dorsad and down- 

 ward in order to take in the last of the left branchial sacs ; vb., buccal cupping 

 glass ; ca., ringed cartilage carrying the principal teeth ; cf., cartilaginous pieces of 

 the face ; I, lingual sucker, (the posterior part of the sucker has not been taken into 

 the section) is shown surrounded by its sheath ; ml., muscles of the lingual sucker ; 

 ph., pharynx ; oe., oesophagus ; m.oe., sphincter closing the entrance of the oeso- 

 phagus ; a.br., branchial aqueduct, showing the seven openings to the branchial 

 sacs ; va., valvular apparatus closing the entrance to the aqueduct ; br s -br., branchial 

 pockets continuing into the coelomic peribranchial cavities, the one being separated 

 from the other by septa ; C, heart : the auricle has been raised partly to show the 

 openings by which it communicates with (1) the ventricle (2) with the sinus 

 venosus, sv., as one sees them both from behind ; vc., entrance of the cardinal veins 

 into the sinus ; j., jugular vein ; vh., hepatic vein ; tao., aortic trunk, with the 

 conus arteriosus and its valvular apparatus at the base of the trunk ; ao., aortic 

 roots, reuniting on a level with the fifth branchial opening to form the aorta ; n., 

 nostril ; sh., bottom of the hypophysial sac with a valvular sac lying before it ; 

 cer., brain ; m., medulla ; cd., dorsal cord ; /, liver ; ov., ovary ; p., posterior cul- 

 de-sac of the branchial enclosure which protects the pericardium. 



Fig. 4. Mouth of the Marine Lamprey, de., teeth of the head of the lingual 

 sucker ; di., lower median tooth ; ap., principal lateral teeth grouped in twos ; da., 

 accessory teeth ; ph, sensory papillae of the buccal lip ; os., cutaneous sensory 

 organs. 



Fig. 5. Anterior region of the skeleton : ca., ringed cartilage carrying th,e 

 principal teeth, d., cf., cartilaginous parts of the face ; cr., brain box ; I, lingual 

 cartilage ; ol, olfactory capsule ; cd., dorsal cord ; an., the two neural arches, anterior 

 and posterior, of the same metamere ; cbr., branchial enclosure ; a, cartilaginous 

 rings surrounding the opening of the external gills ; p, posterior cul-de-sac of the 

 branchial enclosure; holding the heart (after Parker) ; the left half only of the 

 skeleton is represented. 



Fig. 6. Section of a horny tooth (odontoid) of lamprey: ep., buccal epithe- 

 lium ; pa., dermal papillae ; D, tooth in use ; d, replacing tooth, in process of 

 development; k, horn producing cellules (After Warren). 



Fig. 7. Sagittal section of the Pineal Eye. : ep., epidermis ; de., dermis ; op., 

 pineal eye ; np., pineal nerve ; pp., parapineal eye ; ha., commissure and habenular 

 ganglion ; ch, chorioid curtain and lamella concealing the mesencephalon ; ca., 

 anterior commissure ; on the walls of the thalamencephalon ; cp., posterior com- 

 missure, cr., cranial cartilage (after Studnicka). 



Fig. 8. Section of a branchial pocket, passing through internal and external 

 openings. (By reason of the situation of these two openings, the section of the 

 left side of the figure is practically on a plane which places the face to the observer 

 and forms an angle of 45 with the median plane of the animal ; the rest of the 

 section, which is only drawn in, is entirely transverse, and therefore seems short- 

 ened), cd, dorsal cord; an, neural arch; m, section of the medulla; g, fatty tissue 

 completing the padding of the neural canal ; oe, oesophagus and beneath, the aorta ; 

 abr, branchial aqueduct ; t.ao., aortic trunk ; I, lingual cartilage and its muscular 

 sheath; j, jugular vein; cae t appendages of the general cavity; cbr., section of the 

 divers pieces of the branchial enclosure ; o, internal branchial orifice at the in- 

 terior of the pocket; o', external branchial opening, with its threefold valves and 

 cartilaginous ring, a.br., wall of the branchial pocket ; fbd, branchial leaves ; par., 

 peribranchial cavity slightly taken in section ; mu, muscles. 



(1) Vignettes of the title: Scheme of the respiratory apparatus of two cy- 

 clostomes seen from the ventral surface : the oesophagus and the respiratory sacs 

 of the left side (G) only are represented; the horizontal flesh is turned toward the 

 caudal end of the animal ; the oesophageo-cutaneous canal, which exists only on the 

 left side is figured in discontinued tracts. To the left the respiratory apparatus of 

 Myxine is seen (six branchial pockets with efferent canals running to a single 

 opening) to the right, the respiratory apparatus of Bdellostoma polytrema (10-14 

 branchial pockets). (After Dean in Goodrich). (From the chart of Remy Per- 

 rier & Cepede). 



