Cetoconcha spinosula, (THIELE, 1912), 2019

CETOCONCHA SPINOSULA ( THIELE, 1912) View in CoL COMB. NOV.

(NEW COMBINATION FOR POROMYA SPINOSULA THIELE )

( FIGS 12, 13)

Description

Shell: Ovate, relatively thick, inflated, equivalve and approximately equilateral; with a brownish periostracum and sand grains covering the shell; without lithodesma.

Mantle: Ventral mantle margin with one anterior and wide pedal gape, extending from the anterior adductor until two-thirds of the full length of ventral margin; posteroventral mantle margin fusion formed by inner folds (Type A) ( Yonge, 1982); without a fourth pallial aperture.

Siphons: Separated, different in size and outline; inhalant siphon large, modified in a raptorial appendage, typically retracted into the infra-septal chamber when the living animal is at rest or, in museum specimens, due to the alcohol contraction; the inverted inhalant siphon can also be referred to in the literature as a siphonal cowl (hood) or branchial valve, the latter considered a misinterpretation ( Pelseneer, 1911; Yonge 1928, Bernard, 1974, Morton, 1981); exhalant siphon short, cone-like; siphons surrounded at the base by a ring of large siphonal tentacles, ten around the inhalant and three around exhalant; presence of ~15 siphonal papillae between the siphonal tentacles.

Septum: Thin, transparent, perforated by three rows of grouped pores without interfilamental connections, comprising six pairs of pores anteriorly, five to six in the middle and four posteriorly. Presence of a swollen, hollow and bilobate sac in the posterior inner floor of the septum.

Labial palps: Non-lamellate and asymmetric with anterior labial palps well-developed, large, cup-shaped and posterior palps reduced, almost imperceptible in the 3D reconstructions.

Musculature: Posterior and anterior adductor muscles present, isomyarian; with posterior and anterior pedal and septal retractor muscles; lateral septal muscles and taenioid muscle absent.

Foot: Large and elongated; without pedal groove and byssal thread.

Digestive system : The funnel-shaped mouth opens into a thick, short and muscular oesophagus that enters the anterodorsal portion of the stomach; no sphincter was observed between oesophagus and stomach opening; stomach large, rounded, with longitudinal deep grooves in the posterodorsal and anteroventral walls; with prey inside the gastric chamber (maybe ostracod and copepods); stomach connected to the short and small crystalline style sac located at the ventroanterior portion of the stomach floor; stomach surrounded dorsally and anteriorly by the gonads and digestive gland; crystalline style sac conjoined with an anterior mid gut; crystalline style not visible; pericardium/heart surround the rectum/hindgut; hindgut passes above the kidney.

Organs of visceral mass: Haemocoel spaces present in the dorsoposterior portion of the visceral mass.

Reproductive system : Hermaphroditic; ovary and testis well visible, closely associated to the digestive gland; ovary dorsally located and testis in the anteroventral portion of the visceral mass, close to the lateral wall of the stomach.

Nervous system : Circum-oesophagic, pedal and visceral ganglia observed.

62 F. M. MACHADO ET AL .