Genotype: Pellibranchus cinnabareus n.sp. Wellington, N.Z.
P. cinnabareus is unique in the family Phyllirhoidae in the relation of the anus and nephroproct, the absence of functional jaws, the thick body wall with uniform ciliation, and in its non-pelagic habit. Within the family it resembles Phylliroe in having the nephroproct in association with the anus but is unlike that genus since these structures in P. cinnabareus are in the dorsal position whereas in Phylliroe the nephroproct is on the right side of the body. The association of the nephroproct and anus is general in the Dendronotaceae (Odhner, 1936), but in the members of the family Phyllirhoidae the nephroproct lies about the middle of the right side and lacks constant relationship with the anus. Accordingly too much significance may not be attached to this feature in the family.
Pellibranchus has no chitinous sclerites in the buccal bulb that could function as jaws in the usual meaning of the word, but covering the bulb internally and externally is a membranous connective tissue sheath, forming round the frontal portion of the true mouth a cone of sclerites usually twelve in number. The writer has not observed that these function as jaws. There is some variation in the shape of the jaws in other members of the family, Cephalopyge being without the usual processus masticatorius of Phylliroe. Odhner (1936) considers this the primitive condition.
Pellibranchus is more nearly allied to Cephalopyge than to Phylliroe both genera having a distinct foot and dorsal anus. In Cephalopyge there are usually three liver caeca, but the number and size is variable. In Pellibranchus the liver is more compact, with three well defined caeca in which the left anterior liver appears dorsal in position, not the right as is the case with the other members of the family. The right hepatic caecum in Pellibranchus is ventrolateral in position and generally obscured in lateral view by the anterior genital mass. (Plate 9.) Odhner (1936) discussing the difference in the liver structure of the closely allied genera Fimbria and Melibe considers that the differences “may be due to the various formation of the gonad which stands in intimate correlation to the liver.” Possibly the great development of the anterior genital mass in Pellibranchus can be correlated with the reduction of the right anterior liver. Slight divergence is seen in the fact that Pellibranchus has bluntly lobed caeca, while in Cephalopyge these structures are simple and tubular, although too much significance may not be attached to this distinction as Hoffman (1906) states that Vesschelli observed some variation in the liver of Phylliroe, where one specimen bore a few small diverticulae and in another they were totally atrophied.
Odhner (1936) further states that “the liver of nudibranchs shows a tendency to disintegrate from a compact to a more or less branched organ, and that the liver may consequently show the same stages of disintegration in different families, as well as different stages within the same family; therefore it is not apt as a direct indicator of relationship.”
The absence of a disc on the head of Pellibranchus further distinguishes it from Cephalopyge, and the distinct nature of the two
genera is established by the following structures of Pellibranchus: the penis is armed with hooked spines; no ampulla is developed on the hermaphrodite duct; an albumen gland is present as well as a mucous gland; separate male and female ducts leave the hermaphrodite gland. At the same time the known species of Pellibranchus superficially resembles C. trematoides, being very similar in size, shape and colour, but the number of gonads in P. cinnabareus is seven, two ventral sperm lobules, and five dorsal ovarian lobules, whereas in C. trematoides there are five hermaphrodite lobules.
Although the genus Pellibranchus is assigned to the family Phyllirhoidae it may be necessary ultimately to create a new family or amend the present family definition to include this genus on account of its unique features, but the writer at present is unwilling to do this in view of the general agreement with the existing genera.