Subshell membrane

In H. nana, the shell material is described as being deposited against the inside of the capsule (204). The shell/capsule is well developed in the Pseudophyllidea, Tetraphyllidea and Trypanorhyncha. It is often poorly developed or absent in the Cyclophyllidea (Fig. 7.1(a)), in which case (especially in the Taeniidae) the embryophore (see below) is thickened and essentially functions as a protective shell . A subshell membrane has been described in some eggs (Fig. 7.1(a)). 

The presence of a subshell membrane has been noted by a number of workers (439, 440, 541), but it is often difficult to see and its embryonic origin is unknown. In H. diminuta, it is probably represented by the cytoplasmic layer (zone I see below). This membrane appears to be relatively impermeable to many substances and is unaffected by proteases, carbohydrases or lipases. It appears to be a mucopolysaccharide-protein complex and may be important in preventing premature hatching, as well as providing back up protection for the egg shell against a hostile external environment. 

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