Materials and methods
Four Cretaceous thylacocephalan species were examined: Protozoea hilgendorfi Dames, 1886; P. damesi Roger, 1946; Pseuderichthus cretaceus Dames, 1886, and Thylacocephalus cymolopos Lange et al. (2001).
Series of possible lattice organs were found in eight specimens of: Protozoea hilgendorfi: [BM: J 469-1, J 622-1; OR 59245 (2)-1, (3)-1, OR 59554 (3). MB: A 693.3. MSNM: i12172, i20556] and one specimen of P. damesi [BM: OR 59672 (4)] (see Table 1).
Similar structures occurred sporadically in some four specimens of P. hilgendorfi: [BM: PI IC 82; In 42507a; OR 59672 (16)] and an additional specimen MSNM: i7586-3, which is of uncertain thylacocephalan affinity.
Further possible lattice organs were noted, but their poor preservation prevents unequivocal identification: P. hilgendorfi: [BM: PI IC 76, 83; In 48832 (2), (3); OR 59554 (2), OR 59672 (14), (15) and (24). MB: A 693.4, 693.5. MSNM: i7586-1]; P. damesi: [BM: IC 91-1; OR 59245 (4)-1, OR 59672 (25).]
A section (indicated in Fig. 1A) of the posterior carapace along the dorsal midline of one especially well-preserved specimen (BM: J 622-1) was carefully removed. The section containing four pairs of possible lattice organs was subjected to standard high voltage SEM (JEOL JSM-35C), and images were saved in the Semaphore digital image storage system.
In addition, specimens of the Jurassic thylacocephalan Dollocaris ingens van Straelen, 1923 were examined. Structures reminiscent of possible lattice organs were noted on two specimens (MNHN: R.50930 and R.50937).
Specimen repositories are abbreviated as follows: BM – The Natural History Museum, London; MB – Museum für Naturkunde, Berlin; MNHN – Museum Nationale d’Histoire Naturelle, Paris; MSNM – Museo Civico di Storia Naturale, Milan.